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.

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?


In GNU Emacs 24.3.50.1 (i686-pc-mingw32)
 of 2013-08-31 on ODIEONE
Bzr revision: 114088 rgm@gnu.org-20130901021232-sazo0qla2qgcko95
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/binary --enable-checking=yes,glyphs
 'CFLAGS=-O0 -g3' LDFLAGS=-Lc:/Devel/emacs/lib
 CPPFLAGS=-Ic:/Devel/emacs/include'