doc: Add more workqueue functions to the documentation

There are some workqueue functions declared in workqueue.h, so include
that in the workqueue section of the DocBook docs.
Signed-off-by: NTim Bird <tim.bird@sonymobile.com>
Acked-by: NRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/DocBook/device-drivers.tmpl

@@ -66,6 +66,7 @@
 !Ekernel/time/hrtimer.c
      </sect1>
      <sect1><title>Workqueues and Kevents</title>
+!Iinclude/linux/workqueue.h
 !Ekernel/workqueue.c
      </sect1>
      <sect1><title>Internal Functions</title>