longjmp.3: Refer the reader to NOTES in setjmp(3)

There the reader will see the just expanded discussion of
why nonlocal gotos can make code harder to maintain.

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

man3/longjmp.3

@@ -137,6 +137,9 @@ they are not declared as
 .P
 Analogous remarks apply for
 .BR siglongjmp ().
+
+See also the NOTES section in
+.BR setjmp (3).
 .SH SEE ALSO
 .BR setjmp (3),
 .BR sigsetjmp (3)