wait.2: BUGS: Document odd waitid() behavior when 'infop' is NULL

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

man2/wait.2

@@ -602,6 +602,21 @@ main(int argc, char *argv[])
     }
 }
 .fi
+.SH BUGS
+According to POSIX.1-2008, an application calling
+.BR waitid ()
+must ensure that
+.I infop
+points to a
+.I siginfo_t
+structure (i.e., that it is a non-NULL pointer).
+On Linux, if
+.I infop
+is NULL,
+.BR waitid ()
+succeeds, and returns the process ID of the waited-for child.
+Applications should avoid relying on this inconsistent,
+nonstandard, and unnecessary feature.
 .SH "SEE ALSO"
 .BR _exit (2),
 .BR clone (2),