qga: improve fsfreeze documentations

Some users find the fsfreeze behaviour confusing. Add some notes about
invalid mount points and Windows usage.

Related to:
https://bugzilla.redhat.com/show_bug.cgi?id=1436976

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
