pidfd_open.2: Document PIDFD_NONBLOCK

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

man2/pidfd_open.2

@@ -49,8 +49,17 @@ the close-on-exec flag is set on the file descriptor.
 .PP
 The
 .I flags
-argument is reserved for future use;
-currently, this argument must be specified as 0.
+argument either has the value 0, or contains the following flag:
+.TP
+.BR PIDFD_NONBLOCK " (since Linux 5.10)"
+.\" commit 4da9af0014b51c8b015ed8c622440ef28912efe6
+Return a nonblocking file descriptor.
+If the process referred to by the file descriptor has not yet terminated,
+then an attempt to wait on the file descriptor using
+.BR waitid (2)
+will immediately return the error
+.BR EAGAIN
+rather than blocking.
 .SH RETURN VALUE
 On success,
 .BR pidfd_open ()
@@ -62,7 +71,7 @@ is set to indicate the error.
 .TP
 .B EINVAL
 .I flags
-is not 0.
+is not valid.
 .TP
 .B EINVAL
 .I pid