Document that std::os::unix::process::CommandExt.uid() triggers setgroups(0,0)

[zackw]

While digging around for something else I have just discovered that setting the UID for a subprocess with CommandExt.uid() also causes a call to setgroups(0,0) in the child. This should be documented.

(Related to #38527)

[steveklabnik]

re-tagging as @rust-lang/libs ; would you like this to appear in the docs, or not? If so, we're happy to do it, but want to make sure to not over-specify.

[joshtriplett]

This seems like something we should document, yes. It's an unusual special-case that people should be aware of.

This also interacts with #72160 .

[joshtriplett]

Ok, this is ready for a PR. I've labeled it accordingly.