Document handling of connection-mode sockets by sendto(2).

sendto(2), sendmsg(2) and sendmmsg(2) return ENOTCONN if a destination
address is specified and the socket is not connected and the socket
protocol does not automatically connect ("implied connect").  Document
that.  Also document the fact that the destination address is ignored
for connection-mode sockets if the socket is already connected.

PR:		245817
Submitted by:	Erik Inge Bolsø <knan-bfo@modirum.com>
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D24530

lib/libc/sys/send.2

@@ -28,7 +28,7 @@
 .\"     From: @(#)send.2	8.2 (Berkeley) 2/21/94
 .\" $FreeBSD$
 .\"
-.Dd January 4, 2019
+.Dd April 27, 2020
 .Dt SEND 2
 .Os
 .Sh NAME
@@ -69,18 +69,35 @@ The
 function
 may be used only when the socket is in a
 .Em connected
-state, while
+state.
+The functions
 .Fn sendto ,
 .Fn sendmsg
 and
 .Fn sendmmsg
-may be used at any time.
+may be used at any time if the socket is connectionless-mode.
+If the socket is connection-mode, the protocol
+must support implied connect (currently
+.Xr tcp 4
+is the only protocol with support) or the socket must be in a
+connected state before use.
 .Pp
 The address of the target is given by
 .Fa to
 with
 .Fa tolen
-specifying its size.
+specifying its size, or the equivalent
+.Fa msg_name
+and
+.Fa msg_namelen
+in
+.Fa struct msghdr .
+If the socket is in a connected state, the target address passed to
+.Fn sendto ,
+.Fn sendmsg
+or
+.Fn sendmmsg
+is ignored.
 The length of the message is given by
 .Fa len .
 If the message is too long to pass atomically through the
@@ -195,6 +212,8 @@ An invalid descriptor was specified.
 The destination address is a broadcast address, and
 .Dv SO_BROADCAST
 has not been set on the socket.
+.It Bq Er ENOTCONN
+The socket is connection-mode but is not connected.
 .It Bq Er ENOTSOCK
 The argument
 .Fa s
@@ -242,6 +261,7 @@ This typically means that the socket
 is not connected.
 .El
 .Sh SEE ALSO
+.Xr connect 2 ,
 .Xr fcntl 2 ,
 .Xr getsockopt 2 ,
 .Xr recv 2 ,