poll.2: The negate-fd-to-ignore technique does not work for file descriptor 0

See https://bugzilla.kernel.org/show_bug.cgi?id=79411

Reported-by: Nadav Har'El <nyh@math.technion.ac.il>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/poll.2

@@ -83,7 +83,8 @@ file descriptor for a single
 .BR poll ()
 call: simply negate the
 .I fd
-field.)
+field.
+Note, however, that this technique can't be used to ignore file descriptor 0.)
 
 The field
 .I events