tag things that require features in their rustdoc

[Lokathor]

The Cargo.toml and lib.rs is already set up for this, we just need to mark individual elements with a doc(cfg()):

Example:

#[cfg_attr(docs_rs, doc(cfg(feature = "alloc")))]

And replace on whatever feature name. This can (I think) go on modules, types, free functions, etc.

Maybe we just need to mark modules and the rest of obvious enough? I guess we don't want to get too spammy with it.