unexpected_cfgs lint doesn't recognize some standard cfgs

[joshlf]

As part of google/zerocopy#1204, I discovered that the unexpected_cfgs lint fires on some standard cfgs that are common in the ecosystem. The two I discovered so far are:

• doc_cfg
• kani (used by the Kani verifier)

I've had to manually emit directives for these. It's not that onerous, but I could imagine it getting onerous especially for projects that don't already have a build.rs script.

It would be good if the lint could be taught about some of the standard cfgs in the ecosystem to cut down on the noise.

[saethlin]

I submitted #124804 to add other well-known cfgs to thev list but it's not clear to me from the lint documentation if there is any interest in accepting additions not used by rustc/cargo, though to my mind accepting cfg(miri) is a clear precedent.

[Urgau]

I think doc_cfg could be replaced by docsrs, it's automatically inserted by docs.rs since rust-lang/docs.rs#2390