Issue Description:

This issue tracks coverage of the educational notes feature. Below is a non-exhaustive list of diagnostics which could benefit from having educational notes.

• Feel free to add new diagnostics to the list if you notice any that are missing

• If you're contributing a new note, please create a subtask for tracking

• After adding a note, mark the item in the list below as completed

Details on how to contribute notes are available at https://github.com/apple/swift/blob/master/docs/Diagnostics.md#quick-start-guide-for-contributing-new-educational-notes

• missing_append_interpolation, append_interpolation_static, append_interpolation_void_or_discardable, append_interpolation_access_control - explain special ExpressibleByStringInterpolation requirements

• could_not_find_value_subscript, cannot_subscript_base - explain how to declare subscripts

• could_not_use_type_member_on_instance - explain what static members are and how they can be used

• could_not_use_type_member_on_protocol_metatype - explain what protocol meta types are

• candidate_inaccessible, init_candidate_inaccessible - intro to access control

• cannot_pass_rvalue_mutating_subelement, cannot_pass_rvalue_mutating, cannot_pass_rvalue_mutating_getter_subelement, cannot_pass_rvalue_mutating_getter, cannot_apply_lvalue_unop_to_subelement, cannot_apply_lvalue_unop_to_rvalue, cannot_apply_lvalue_binop_to_subelement, cannot_apply_lvalue_binop_to_rvalue - explain mutating keyword

• expression_too_complex - give a very high level overview of why this happens, plus some tips for solving

• cannot_pass_rvalue_inout_subelement, cannot_pass_rvalue_inout_converted, inout_change_var_type_if_possible, cannot_pass_rvalue_inout, cannot_provide_default_value_inout - explain inout argument restrictions

• cannot_convert_thrown_type, cannot_throw_error_code - how to write an Error-conforming type that can be thrown

• cannot_pass_type_to_non_ephemeral, cannot_pass_type_to_non_ephemeral_warning, cannot_use_inout_non_ephemeral, cannot_use_inout_non_ephemeral_warning, cannot_construct_dangling_pointer, cannot_construct_dangling_pointer_warning - explain @_nonEphemeral diagnostics, possible solutions to get proper behavior

• throws_functiontype_mismatch - explain throws effect on function subtyping

• serialization_module_too_new, serialization_module_language_version_mismatch, serialization_module_too_old - explain why library evolution is required to use modules compiled w/ different compilers

• unspaced_binary_operator_fixit, unspaced_binary_operator, unspaced_unary_operator - explain operator whitespace rules

• module_not_compiled_with_library_evolution - explain why imports require library evolution

• declared_operator_without_operator_decl - Explain operator decls and why just an implementation isn't sufficient

• declared_unary_op_without_attribute, unary_op_missing_prepos_attribute, invalid_arg_count_for_operator, operator_in_local_scope, nonstatic_operator_in_nominal, nonstatic_operator_in_extension, nonfinal_operator_in_class, operator_in_unrelated_type - operator implementation requirements

• optional_not_unwrapped, optional_base_not_unwrapped - explain pros and cons of different optional unwrapping strategies

• missing_forced_downcast - explain the narrow cases where this is ok, and how it may just be programmer error

• missing_explicit_conversion - explain why this is needed for e.g. bridging

• invalid_c_function_pointer_conversion_expr, c_function_pointer_from_method, c_function_pointer_from_generic_function - explain c function ptr conversion rules

• attribute_requires_operator_identifier - explain allowable chars in operator identifier

• invalid_iboutlet, iboutlet_nonobjc_class, iboutlet_nonobjc_protocol, iboutlet_nonobject_type, iboutlet_only_mutable, iboutlet_non_optional - explain @IBOutlet requirements

• invalid_ibdesignable_extension - @IBDesignable restrictions

• inherited_default_value_not_in_designated_constructor, inherited_default_value_used_in_non_overriding_constructor, corresponding_param_not_defaulted - explain designated inits

• pattern_type_not_usable_from_inline, pattern_type_not_usable_from_inline_warn, pattern_type_not_usable_from_inline_frozen, pattern_type_not_usable_from_inline_inferred, pattern_type_not_usable_from_inline_inferred_warn, pattern_type_not_usable_from_inline_inferred_frozen - explain @usableFromInline

• Explain access control diagnostics (many)

• opaque_type_invalid_constraint, inferred_opaque_type - explain opaque return types

• Explain protocol conformance diagnostics (many)

• Explain witness matching diagnostics (many)

• Explain associated type inference diagnostics (many)

• Explain override diagnostics (many)

• multiple_enum_raw_types, raw_type_not_first, raw_type_not_literal_convertible, enum_raw_type_not_equatable, enum_raw_type_nonconforming_and_nonsynthable, empty_enum_raw_typeenum_raw_value_without_raw_type, enum_with_raw_type_case_with_argument, enum_non_integer_raw_value_auto_increment, enum_non_integer_convertible_raw_type_no_value, enum_raw_value_not_unique - enum raw types

• objc_enum_no_raw_type, objc_enum_raw_type_not_integer - @objc enum raw values

• Explain Hashable derivation diags

• Explain Equatable derivation diags

• Explain coddle derivation diags

• Explain S4TF AdditiveArithmetic derivation requirements

• dynamic_self_invalid, dynamic_self_in_mutable_property, dynamic_self_in_stored_property, dynamic_self_in_mutable_subscript, dynamic_self_invalid_property, dynamic_self_invalid_subscript, dynamic_self_invalid_method, dynamic_self_stored_property_init, dynamic_self_default_arg - allowable uses of covariant Self

• autoclosure_function_type, invalid_autoclosure_and_convention_attributes, autoclosure_function_input_nonunit - explain how @autoclosure works

• Explain lazy var diagnostics (many)

• Explain S4TF @differentiable attribute diagnostics (many)

• Explain S4TF @Derivative attribute diagnostics (many)

• Explain convenience init diagnostics (many)

• optional_used_as_boolean, integer_used_as_boolean - explain conditions must be boolean, suggest alternatives

• failed_to_produce_diagnostic - link to bugs.swift.org and explain how to file a bug (maybe add this to type checker in reasonable time, protocol x is broken diagnostics too)

• Explain unsafeBitCast diagnostics (many), including why it should almost never be used

• Explain @NoEscape diagnostics (many)

• return_non_failable_init, return_init_non_nil - explain mailable inits

• no_match_operator - explain how to implement custom pattern matching

• unknown_case_must_be_catchall, unknown_case_where_clause, unknown_case_multiple_patterns, unknown_case_must_be_last - explain @unknown attribute

• Explain do-try-catch diagnostics (many)

• Explain required init diagnostics

• Explain @objc diagnostics (many)

• Explain @available diagnostics (many)

• Explain exclusivity violation diagnostics (many)

• Explain definite initialization diagnostics (many)

• Explain overflow diagnostics (many)