docs: Clarify distinction between GDrive, GVolume and GMount

Provide examples of what they all represent, and expand on the descriptions of them in a few places.

Move references to their equivalents from GnomeVFS to lower down in the documentation, since GnomeVFS has been deprecated for many years now, and is unlikely to be pertinent to most readers.

Signed-off-by: Philip Withnall pwithnall@gnome.org