refine comments for librtmp

@@ -1059,6 +1059,7 @@ typedef void* srs_hijack_io_t;
     /**
      * writev of socket.
      * @return 0, success; otherswise, failed.
+     * @remark We assume that the writev always write all data to peer, like what ST or block-socket done.
      */
     extern int srs_hijack_io_writev(srs_hijack_io_t ctx, const iovec *iov, int iov_size, ssize_t* nwrite);
     /**
@@ -1075,6 +1076,7 @@ typedef void* srs_hijack_io_t;
     /**
      * write bytes to socket.
      * @return 0, success; otherswise, failed.
+     * @remark We assume that the write always write all data to peer, like what ST or block-socket done.
      */
     extern int srs_hijack_io_write(srs_hijack_io_t ctx, void* buf, size_t size, ssize_t* nwrite);
 #endif