bug#22314: 25.1.50; Document variable `deactivate-mark' in Elisp manual

[Drew Adams <drew.adams@oracle.com>]

> > I think you are.  At least for me, with the latest Emacs 25
> > snapshot I have (2015/12/10), `i deactivate-mark' shows two
> > index entries, and `,' bounces between them.  They are:
> > (1) deactivate-mark, the function, and (2) deactivate-mark-hook,
> > a variable.  Variable `deactivate-mark' is not one of them.
> >
> > Do you really see something different?
> 
> I see the same, but I don't understand why that is a problem.  The
> function and the variable are described one after the other, and 'i'
> puts you on the first of them with the second clearly visible below.
> How is that a problem?  And how is it worse than having 2 identical
> index entries instead, which point each one to a place several lines
> apart?

It's wrong because it does not move point to the entry.  Nothing
indicates to a user that there in fact 3 entries, not 2.  Whether
you might happen to have the 3rd visible in the same window (and
you might not, depending on your window size), you might well not
notice it there - as I did not.

I would not have filed this bug report if I thought that this
was not a problem.  And as you can see from my initial report,
I in fact mistakenly thought that the variable was not even
documented, because cycling among the index entries did not
take me to it.

I don't see why you wouldn't want to add an index entry for this
variable.  But if you don't feel like it then what can I say?

If the Elisp manual had different indexes, as does the Emacs
manual, then adding it would also let a user find it in the
Variables Index.

Maybe it's not possible to index both, if there is only one
Index?  Dunno.  If you can't, you can't.  If you can (maybe two
entries, with suffixes "(variable)" and "(function)"), that's
better, IMO.