getgroups.2: ERRORS: add EPERM for the case where /proc/PID/setgroups is "deny"

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

man2/getgroups.2

@@ -133,6 +133,15 @@ The calling process has insufficient privilege
 (it does not have the
 .BR CAP_SETGID
 capability).
+.TP
+.B EPERM
+The use of
+.BR setgroups ()
+is denied in this user namespace.
+See the description of
+.IR /proc/[pid]/setgroups
+in
+.BR proc (5).
 .SH CONFORMING TO
 SVr4, 4.3BSD.
 The