bug#8614: 24.0.50; doc of `deactivate-mark'

[Drew Adams]

> > The doc for this function is now woefully inadequate.  The 
> > function does much, much more nowadays than is hinted in the
> > doc.  Please update the doc to describe ALL of the many things
> > that this function does, including conditional behavior that
> > depends on the current context, including various variables.
> 
> Do you have a suggestion for a new doc string?

Sorry, I'm not the expert on what it does.  But it is clear to even me that it
does much more than is suggested in the doc.

If someone comes up with a complete list of what to say about what it does then
I can suggest wording.  Someone knowledgable needs to come up with the content.

"Someone knowledgable" could include those who have changed its behavior most
recently.  When the behavior is extended and becomes more complex, so can the
doc need to be updated.