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

[Eli Zaretskii]

> Date: Fri, 13 Sep 2013 15:17:31 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> 
> (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.