Issue Description:

The documentation for `LazySequenceProtocol` Overview section includes some sample code for building "a sequence that lazily computes the elements in the result of scan". As written, this example code does not compile under Swift 5.0 or 5.1. This documentation comes from comments in stdlib/public/core/LazySequence.swift .

In particular, the `LazyScanIterator` construction in `LazyScanSequence::makeIterator()` gets the following compiler errors:

• "LazyScanIterator<Base, ResultElement>' initializer is inaccessible due to 'private' protection level"

• "Missing argument label 'nextPartialResult:' in call"

Similarly for the `LazyScanSequence` construction in `LazySequenceProtocol::scan(…)`:

• "Cannot invoke initializer for type 'LazyScanSequence<_, _>' with an argument list of type '(initial: ResultElement, base: Self, (ResultElement, Self.Element) -> ResultElement)'"

Since readers of the documentation are likely to want to copy and paste this code and try it out, or copy it when making their own `LazySequenceProtocol` conforming struct, this code should compile and run properly.

A PR fixing the problems has been provided: #25995