Issue Description:

Related forum thread.

For example this:

typealias Map = Dictionary

is a short hand form for:

typealias Map<K: Hashable, V> = Dictionary<K, V>

according to this forum post by @slavapestov.

But afaict there doesn't seem to be any mention of this in any documentation.

So I guess it should be documented in eg this section:
https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#ID361

And more generally, a lot of the behavior of type aliases, associated types and extensions is both undocumented and inconsistent/broken.

Properly documenting the intended behavior would certainly help not only us (new and seasoned) users but also the compiler devs.