Add documentation for additional reason why snapshot-revert might need
to be forced. This explains why restoring an inactive snapshot while
there is managed saved state is refused by default.
Signed-off-by: Michael Weiser <michael.weiser@gmx.de>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>