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: Tim Bird <tim.bird@sonymobile.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>