net: Document xfrm4_gc_thresh and xfrm6_gc_thresh

This change adds documentation for xfrm4_gc_thresh and xfrm6_gc_thresh
based on the comments in commit eeb1b733 ("xfrm: Increase the garbage
collector threshold").

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>