fix up security_socket_getpeersec_* documentation

Update the security_socket_peersec documentation in
include/linux/security.h.  security_socket_peersec has been split
into two functions - _stream and _dgram, with new capabilities.

Signed-off-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>