Docs: add fs/eventpoll to docbooks

Add fs/eventpoll.c to the filesystem api-summary book.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/r/20210210042526.23174-2-rdunlap@infradead.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>