Rustdoc: Omit default generic arguments

[trolleyman]

I think it'd be nice to omit generic arguments that are equal to the default. This would be mainly to stop things like Box<T, Global> in the std docs, which I think leads them to be unnecessarily noisy.

The strange thing is, it happens inconsistently - sometimes the argument is given and sometimes it isn't.

e.g.: https://doc.rust-lang.org/stable/std/boxed/struct.Box.html

[sfackler]

Duplicate of #80379

[trolleyman]

Thanks, sorry for the duplication.