Allow expanding tidy check for undocumented unsafe code

[pierwill]

This will make it easier to expand this check to e.g. alloc in the future. (See #92485.)

Also adds some comments to the code, and fixes outdated reference to libcore.

r? @the8472

[the8472]

This doesn't seem to have much value on its own. I mean there's probably some desire to get those comments added to alloc and std but that's a bit larger effort.

Maybe ask the libs team first. If they agree that it's a generally desirable goal then an issue should be created to organize the work and this could be the first commit of several.