Document `#![feature(..)]` attribute?

[traviscross]

This is a valid program on stable Rust:

#![feature()]
#![expect(unused_attributes)]
fn main() {}

Consequently, perhaps we should document the feature attribute in the way we do other attributes. Currently, as best as I see, we mention it in our attribute index but do not otherwise really document it.

cc rust-lang/fls#549 @Jules-Bertholet @ehuss