TTY, devpts: document pty count limiting

Logic has been changed in kernel 3.4 by commit e9aba5158a
("tty: rework pty count limiting") but still not documented.

Sysctl kernel.pty.max works as global limit, kernel.pty.reserve ptys
are reserved for initial devpts instance (mounted without "newinstance").
Per-instance limit also could be set by mount option "max=%d".

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/sysctl/kernel.txt

@@ -64,6 +64,7 @@ show up in /proc/sys/kernel:
 - printk_delay
 - printk_ratelimit
 - printk_ratelimit_burst
+- pty                         ==> Documentation/filesystems/devpts.txt
 - randomize_va_space
 - real-root-dev               ==> Documentation/initrd.txt
 - reboot-cmd                  [ SPARC only ]