WIP: Documentation for Publishing Changes

suggest bringing up front that the control that these features give you fills different needs:

- at the section and subsection level through release dates, and at the unit level through publishing status and the hide from students option

- to make content available automatically on a predefined schedule (rel dates), to make content available manually only when a unit's content is complete, and to remove content from the course ad hoc (hide) until and if you are ready to reinstate it