Implement username and groupname lookup for account switching

[myrrlyn]

This commit implements named user/group lookup, and updates the documentation to reflect that this is now possible.

It also has some unrelated documentation touchups; I can strip that if it's undesired.

[vorner]

Hello

Thank you :-)

In general, it is slightly better to submit more merge requests if the changes are unrelated. But it's probably not worth the effort splitting it now, so I'm just merging it.

I'll make a release once I get to a proper Internet connection.