[rustdoc] Private macros are not documented with `--document-private-items`

[TyPR124]

Rustdoc has --document-private-items and --document-hidden-items. It would be nice to have an option to document non-exported macros, which is currently not possible as far as I can tell.

[jonas-schievink]

I would expect --document-private-items to also document macros, so this seems like a bug

[jyn514]

This is because rustdoc gets macros from Crate.exported_macros, which doesn't have the private macros in the first place. Intra-doc links don't have this problem because they look at Resolver::all_macros. Maybe the rest of rustdoc could use that too?

[camelid]

Marking as P-medium since this behavior is pretty confusing.

[camelid]

Going to try working on this. Hopefully I won't regret it ;)