bug#15375: 24.3.50; Elisp manual: mention what units are used for window margin functions

[Drew Adams]

> > (elisp) `Display Margins` talks a lot about window margins, but AFAICT
> > it never once says which units are used for the functions and variables
> > described there.  I'm guessing the unit is `frame-char-width', but Emacs
> > should tell us explicitly here.
> 
> The units are character cells (a.k.a. "columns").
> 
> I fixed this in trunk revision 114279.
> 
> > Moreover, we don't even tell users what a nil value means, as opposed
> > to a whole number.
> >
> > For example, right now, `M-: (window-margins)' returns `(nil)' for me,
> > which corresponds to both LEFT and RIGHT being nil.  What does a nil
> > value mean for each of these?
> 
> Fixed that as well.

Thanks.  Do you want me to close the bug, or do you want to?
(Dunno whether you wanted to keep it open for some reason.)