apparmor: update kernel doc comments for xxx_label_crit_section

Add a kernel doc header for __end_current_label_crit_section(), and
update the header for __begin_current_label_crit_section().

Fixes: b42ecc5f58ef ("apparmor: make __begin_current_label_crit_section() indicate whether put is needed")
Signed-off-by: John Johansen <john.johansen@canonical.com>