rhashtable: Documentation tweak

Clarify that rhashtable_walk_{stop,start} will not reset the iterator to
the beginning of the hash table.  Confusion between rhashtable_walk_enter
and rhashtable_walk_start has already lead to a bug.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>