document the meaning of protocol = 0.

diff --git a/lib/libc/sys/socket.2 b/lib/libc/sys/socket.2
index 364ab4a..858287b 100644 (file)
--- a/lib/libc/sys/socket.2
+++ b/lib/libc/sys/socket.2
@@ -116,6 +116,10 @@ must be specified in this manner.  The protocol number to use is
 particular to the \*(lqcommunication domain\*(rq in which communication
 is to take place; see
 .Xr protocols 5 .
+A value of 0 for
+.Fa protocol
+will let the system select an appropriate protocol for the requested
+socket type.
 .Pp
 Sockets of type
 .Dv SOCK_STREAM