Rustdoc: always render required features, even if they are default

[jaskij]

As best I can tell, currently Rustdoc only renders the "Availabe on crate feature xxx only." box if this feature is not default.

For various reasons, a library user may want to cut down and use default-features = false in their Cargo.toml (in fact, IMO it's good practice for libraries). In those cases, if something is gated behind one of the default features, it leads to confusion, and makes it hard to discover which features are required for a part of a crate to be present.

What I would like to see, is for the required feature box to be always present, even if something is gated behind a default feature.