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

[Eli Zaretskii]

> Date: Sat, 14 Sep 2013 09:26:42 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> Cc: 15375@debbugs.gnu.org
> 
> 1. Move the "(a.k.a.@: ``column'')" to the first occurrence of "character
> cell".  I, for one, did not know the latter term, and wondered about it
> until I got to that aka.
> 
> 2. You can remove this part, as it is implied: "; if none of the two margins
> exist, the function returns @code{(nil)}."  (It does not hurt, but in that
> case please change "none" to "neither", to be clearer.)
> 
> 3. You should add a comma before "and" in "The argument @var{left} controls
> the left margin and @var{right} controls the right margin (default @code{0})."
>                ,
> Alternatively, you can use ";" instead of ", and", or split it into two
> sentences.

Done.