select.2: Mention the 'fd_set' size limitation early and refer to poll(2)

Change this because of the serious limitation of select()
imposing a limit on the range of file descriptors that can
be monitored. This is currently mentioned too late in the
documentation (in the NOTES section). The man page should
warn early and refer to poll(2) as soon as possible.

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/select.2

@@ -91,6 +91,13 @@ perform a corresponding I/O operation (e.g.,
 without blocking, or a sufficiently small
 .BR write (2)).
 .PP
+The file descriptors
+.BR select ()
+can monitor cannot be larger than
+.B FD_SETSIZE.
+.BR poll (2),
+does not have this limitation.
+.PP
 The operation of
 .BR select ()
 and