ioctl_ns.2: ERRORS: document ENOTTY

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

man2/ioctl_ns.2

@@ -136,6 +136,16 @@ or
 in order to create a namespace.
 .SH CONFORMING TO
 Namespaces and the operations described on this page are a Linux-specific.
+.SH ERRORS
+Any of the above
+.BR ioctl ()
+operations can return the following errors:
+.TP
+.B ENOTTY
+.I fd
+does not refer to a
+.I /proc/[pid]/ns/*
+file.
 .SH EXAMPLE
 The example shown below uses the
 .BR ioctl (2)