fix: docs and cfg std for select macro

[tisonkun]

This closes #2932

cc @taiki-e @petekubiak

[taiki-e]

Thanks for the PR, but this is not correct. select! refers random module which gated under std feature, so cannot be used without std feature.

The correct action here is to make select no-std compatible (there are already two PRs) or to mention features enabled by default in docs (or issue doc(cfg)).

[tisonkun]

@taiki-e Thanks for your information! I'll take a look later.

Thanks for the PR, but this is not correct. select! refers random module which gated under std feature, so cannot be used without std feature.

I tried to run:

cargo build --no-default-features --features async-await
cargo build --no-default-features --features async-await-macro

locally. And both are compiled. But yes now I see: __futures_crate::async_await::shuffle(&mut __select_arr); that depends on std.

[tisonkun]

Cross ref:

• no-std select! #1912
• feat: select! macro for no-std environments  #2903