fcntl.2: Rework description of F_SETOWN

As suggested by Jason, make it clearer that I/O signalling
requires the use of both F_SETOWN and O_ASYNC. While we're at,
make a few other cleanups to the text.

Reported-by: Jason Vas Dias <jason.vas.dias@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/fcntl.2

@@ -681,9 +681,9 @@ Set the process ID or process group ID that will receive
 .B SIGIO
 and
 .B SIGURG
-signals for events on file descriptor
-.IR fd
-to the ID given in
+signals for events on the file descriptor
+.IR fd .
+The target process or process group ID is specified in
 .IR arg .
 A process ID is specified as a positive value;
 a process group ID is specified as a negative value.
@@ -693,22 +693,23 @@ Most commonly, the calling process specifies itself as the owner
 is specified as
 .BR getpid (2)).
 
-.\" From glibc.info:
-If you set the
-.B O_ASYNC
-status flag on a file descriptor by using the
+As well as setting the file descriptor owner,
+one must also enable generation of signals on the file descriptor.
+This is done by using the
+.BR fcntl ()
 .B F_SETFL
-command of
-.BR fcntl (),
-a
+command to set the
+.B O_ASYNC
+file status flag on the file descriptor.
+Subsequently, a
 .B SIGIO
 signal is sent whenever input or output becomes possible
-on that file descriptor.
+on the file descriptor.
+The
+.BR fcntl ()
 .B F_SETSIG
-can be used to obtain delivery of a signal other than
+command can be used to obtain delivery of a signal other than
 .BR SIGIO .
-If this permission check fails, then the signal is
-silently discarded.
 
 Sending a signal to the owner process (group) specified by
 .B F_SETOWN
@@ -717,6 +718,8 @@ is subject to the same permissions checks as are described for
 where the sending process is the one that employs
 .B F_SETOWN
 (but see BUGS below).
+If this permission check fails, then the signal is
+silently discarded.
 
 If the file descriptor
 .I fd