Doc: update the documentation of QStandardPaths to be more thorough

Specifically, note what paths can be empty and which ones can't (modulo
system mis-configuration, like an empty $HOME var) and describe what
implications there are for application-specific paths and for global
(user) paths.

Change-Id: If6c11aab466ba50f3a9685dce52dd51b86426f27
Reviewed-by: David Faure <david.faure@kdab.com>