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

Documentation/filesystems/api-summary.rst

@@ -122,6 +122,12 @@ Events based on file descriptors
 .. kernel-doc:: fs/eventfd.c
    :export:
 
+eventpoll (epoll) interfaces
+============================
+
+.. kernel-doc:: fs/eventpoll.c
+   :internal:
+
 The Filesystem for Exporting Kernel Objects
 ===========================================