Question
I want to emerge a certain package, let's say x11-base/xorg-drivers,
so I try first "emerge --pretend xorg-drivers" and find it has ~50
various use-flags (some set, some unset). Where can I find their
description? For example "vmmouse", what is this USE flag good for?
Is it something for vmware?

I checked /etc/portage/profiles/use.desc and use.local.desc but
there are not all of them...

Answer
equery u x11-base/xorg-drivers

equery is part of app-portage/gentoolkit in case you don't have it installed. It can do other useful things such as tell you which package installed a file, or show all files installed by a package.

And in the specific case of xorg-drivers those aren't USE flags at all
but driver flags