Unstable (de facto internal) library feature `gen_future` links to a closed tracking issue (for async/await)

[ArekPiekarz]

Unstable Book in section gen_future leads to a closed tracking issue #50547 titled Tracking issue for async/await (RFC 2394).

There are a few solutions to this:
- reopen the tracking issue, or
- create a new tracking issue and a PR updating the issue number in documentation, or
- mark the feature as internal only, or
- something else?

[fmease]

This is a de facto internal library feature that gates the internal ResumeTy & get_context used in the desugaring of async blocks as far as I can tell.

The solution is easy, simply replace the four occurrences of issue = "50547" in library/core/src/future/mod.rs with issue = "none". This way, the Unstable Book will automatically switch to the synopsis “This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.”.

[progressive-galib]

@fmease done

[progressive-galib]

@rustbot claim