Doc: enhance required feature highlight on the api items

[Mathieu-Lala]

Before :

After :

Summary of changes

I made those changed only in lib.rs but I suppose it should be done on every public item with a #[cfg(feature ...)]

[schungx]

I vaguely remember that rustdoc will eventually put these feature flag tags onto APIs automatically... And so I have been waiting for the feature to land.

As it stands, I think the syntax is quite ugly and a mouthfu... not sure if we want to sprinkle those all across the codespace because if we do it, we probably should add it to all the public API as you mention.

Which actually is not a big deal. A lot of work, but just boilerplate type of work. When the auto feature lands, we can just do a global search and replace it with an empty line.

[Mathieu-Lala]

See
rust-lang/cargo#8103
rust-lang/rust#96318