docs claim `Mutex` is "Available on *crate feature* `alloc` only." but in fact it requires `alloc` *and* `std` (std implies alloc)

[tamird]

https://docs.rs/futures/latest/futures/lock/struct.Mutex.html

alloc required here:

futures-rs/futures-util/src/lib.rs

Lines 322 to 323 in de9274e

	#[cfg(feature = "alloc")]
	pub mod lock;

std required here:

futures-rs/futures-util/src/lock/mod.rs

Lines 26 to 27 in de9274e

	#[cfg(feature = "std")]
	mod mutex;

[taiki-e]

futures-util doc displays it correctly (since std implies alloc, it is better to just display that std is required though), so it seems to be an issue related to re-exporting.

Inlining this re-export manually (like some other modules e.g., executor do) will likely resolve it.