workflows: documentation enrichment
workflows: documentation enrichment

* Enriches the documentation of the Workflows module by adding
  more information on actions and the Holding Pen.

Signed-off-by: Jan Aage Lavik <8d55e006a1b29770f275a5c59a7341f17f8e69d5@cern.ch>

tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio,inveniosoftware/invenio