send.2: Add details on various 'msghdr' fields

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/send.2

@@ -239,10 +239,9 @@ data.
 .SS sendmsg()
 The definition of the
 .I msghdr
-structure follows.
-See
-.BR recv (2)
-and below for an exact description of its fields.
+structure employed by
+.BR sendmsg ()
+is as follows:
 .in +4n
 .nf
 
@@ -253,11 +252,28 @@ struct msghdr {
     size_t        msg_iovlen;     /* # elements in msg_iov */
     void         *msg_control;    /* ancillary data, see below */
     size_t        msg_controllen; /* ancillary data buffer len */
-    int           msg_flags;      /* flags on received message */
+    int           msg_flags;      /* flags (unused) */
 };
 .fi
 .in
 .PP
+The
+.I msg_name
+field is used on an unconnected socket to specify the target
+address for a datagram.
+It points to a buffer containing the address; the
+.I msg_name
+field should be set to the size of the address.
+For a connected socket, these fields should be specified as NULL and 0,
+respectively.
+
+The
+.I msg_iov
+and
+.I msg_iovlen
+fields specify scatter-gather locations, as for
+.BR writev (2).
+
 You may send control information using the
 .I msg_control
 and
@@ -268,10 +284,13 @@ per socket by the value in
 .IR /proc/sys/net/core/optmem_max ;
 see
 .BR socket (7).
+
+The
+.I msg_flags
+field is ignored.
 .\" Still to be documented:
 .\"  Send file descriptors and user credentials using the
 .\"  msg_control* fields.
-.\"  The flags returned in msg_flags.
 .SH RETURN VALUE
 On success, these calls return the number of bytes sent.
 On error, \-1 is returned, and