Omit ambassador macros from docs

[vigna]

Would it be possible, maybe with an option, to have #[doc(hidden)] for ambassador implementation macros? We have quite a few and they clutter the documentation page (I assume the crate user should never know about them).

[dewert99]

I assume the crate user should never know about them

This is not necessarily true, for public delegatable traits users can also use the macros to delegate them for there own structs, for private delegatable traits the macros will also be private and so they shouldn't get documented.