setreuid.2: Error checking should always be performed, even when caller is UID 0

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

man2/setreuid.2

@@ -95,6 +95,13 @@ On success, zero is returned.
 On error, \-1 is returned, and
 .I errno
 is set appropriately.
+
+.IR Note :
+there are cases where
+.BR setreuid ()
+can fail even when the caller is UID 0;
+it is a grave security error to omit checking for a faulure return from
+.BR setreuid ().
 .SH ERRORS
 .TP
 .B EAGAIN