Docbook: add fs/eventfd.c and fix typos in it

Add fs/eventfd.c to filesystems docbook.
Make typo corrections in fs/eventfd.c.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/DocBook/filesystems.tmpl

@@ -82,6 +82,11 @@
      </sect1>
   </chapter>
 
+  <chapter id="fs_events">
+     <title>Events based on file descriptors</title>
+!Efs/eventfd.c
+  </chapter>
+
   <chapter id="sysfs">
      <title>The Filesystem for Exporting Kernel Objects</title>
 !Efs/sysfs/file.c