Documentation for async/await support (in 0.2)

[Nemo157]

Some issues I have noticed with the current documentation when building with --features nightly:

• (Hide __rt from documentation #928) futures::__rt is visible, this should be hidden
• async etc. macros are only in the prelude, for documentation (and people that avoid preludes) I believe these should also be exported from the root (or maybe some other submodule? module name bikeshed welcome)
• No mention of what nightly features are required to use async, should include basics from futures-await.md
• (Drop support for non-immovable #[async] macros #964) Consistency of attributes (Name consistency of async function attributes #918)
• The macros appear as re-exports instead of items (rustdoc: Macros re-exported via use_extern_macro appear as a re-export rust#49554)
• async* macros links from futures::prelude don't work (rustdoc: Bad handling of proc_macro's re-exported via use_extern_macros rust#49553)
• (WIP branch) async* macros have no documentation
• (Add documentation to futures-macro-await macros #930) await! documentation is internal implementation details, should be user facing
• (Add documentation to futures-macro-await macros #930) await_item! documentation is lacking
• (Add documentation to futures-macro-await macros #930) stream_yield! is missing documentation

[Nemo157]

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