fork.2: Add ENOMEM error for PID namespace where "init" has died

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

man2/fork.2

@@ -237,6 +237,12 @@ See
 .BR fork ()
 failed to allocate the necessary kernel structures because memory is tight.
 .TP
+.B ENOMEM
+An attempt was made to create a child process in a PID namespace
+whose "init" process has terminated.
+See
+.BR pid_namespaces (7).
+.TP
 .B ENOSYS
 .BR fork ()
 is not supported on this platform (for example,