net: Document xfrm4_gc_thresh and xfrm6_gc_thresh
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 eeb1b73378b56 ("xfrm: Increase the garbage
collector threshold").

Signed-off-by: Alexander Duyck <af56634f9be8457194a3461ec96c497f138af4ff@redhat.com>
Signed-off-by: Steffen Klassert <66deb1d63f85161cc287ce2bac0c65cb5ae958a7@secunet.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs