Move keyword page makes no mention of async blocks

[That3Percent]

The move keyword page currently talks only about the use of move with closures. The move keyword is also valid before an async block. For example: async move { /*...*/ }. This page could be updated to include that usage.

This issue has been assigned to @Pulkit07 via this comment.

[nikomatsakis]

Mentoring instructions

Edit src/libstd/keyword_docs.rs and make the text talk about async blocks as well.

[Pulkit07]

@rustbot claim