cargo doc: add option to document only public items for binary crates

[dima74]

Describe the problem you are trying to solve

#7593 enables --document-private-items flag by default for binary crates. As stated in the PR:

We should also probably add a --document-only-public-items flag or something like that if we change the default behavior. Otherwise users have no way to not document private items for binary crates. Right? -> We can do it later

Describe the solution you'd like

Add flag like --document-only-public-items which can be used for binary crates to disable flag --document-private-items (which is enabled by default)

[Qix-]

Took a whack at this in #10979. Hopefully that solves the need :)

[tom25519]

Hi, just wanted to bump this, as it's definitely still something I would find useful. Generating documentation for a binary package can be useful to someone navigating a codebase for the first time, but unfortunately, certain macros tend to leave undocumented private items all over a crate (e.g: Tracing leaves private CALLSITE and META statics wherever any of its macros are used). The reasoning for closing #10979 was that #[doc(hidden)] can be used, but obviously in such cases, this is not possible. Plus tagging everything with #[doc(hidden)] gets kind of annoying anyway. Would the Cargo team be interested in merging a PR to address this?