pipe.2: Add mention of O_NOTIFICATION_PIPE flag

pipe.2: Add mention of O_NOTIFICATION_PIPE flag

This adds very basic information about the notification pipe that have
been added into Linux 5.8.

There is some description about the interface at:

https://www.kernel.org/doc/html/latest/core-api/watch_queue.html

(I think that there is at least on bug in that page, since the
notification pipe has to be opened with O_NOTIFICATION_PIPE which is
defined to O_EXCL not O_TMPFILE)

The ENOPKG error should be clear from this header (see
watch_queue_init() at the end):

https://github.com/torvalds/linux/blob/5bfc75d92efd494db37f5c4c173d3639d4772966/include/linux/watch_queue.h

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
Cc: David Howells <dhowells@redhat.com>
[alx: wsfix]
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>