(srtp_create): Document that policy argument may be NULL.
(srtp_create): Document that policy argument may be NULL.

yarrcc/libsrtp-ios,cuijinquan/libsrtp,ksophocleous/libsrtp,cuijinquan/libsrtp,persmule/libsrtp,Distrotech/libsrtp,persmule/libsrtp,yarrcc/libsrtp-ios,cuijinquan/libsrtp,persmule/libsrtp,ksophocleous/libsrtp,yarrcc/libsrtp-ios,Distrotech/libsrtp