Select macro docs are incorrect

[petekubiak]

Hi, the docs state that the select! macro is gated by the async-await feature, however in the code it is in fact gated by std. Can either the docs be updated or the #[cfg(feature = ...)] in the code?

My hope is that the code is wrong, as I'd very much like to be able to use this in a no_std environment.

[ChocolateLoverRaj]

I did not look at the source code for select!, but my guess is that maybe its std requirement could be changed to alloc instead.