bug#10206: 24.0.92; doc of `window-system-default-frame-alist'

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

The variable's value is not described.  All we say is that it is an
alist that somehow has to do with frame parameters.
 
Please document the alist entries: what is the form of the KEY of an
entry? the VALUE?  In particular, what values are possible for the KEY?
 
In Emacs 23, the doc string was more helpful.  It said this:
 
"Alist of window-system dependent default frame parameters.
You can set this in your init file; for example,
 
 ;; Disable menubar and toolbar on the console, but enable them under X.
 (setq window-system-default-frame-alist
       '((x (menu-bar-lines . 1) (tool-bar-lines . 1))
         (nil (menu-bar-lines . 0) (tool-bar-lines . 0))))
 
Parameters specified here supersede the values given in
`default-frame-alist'."
 
But even that begs the question: What are `x' and `nil' here?  What are
the possible values for an alist entry KEY?
 
Also, in Emacs 23 we explicitly suggest that you use this in your init
file.  In Emacs 24 that has been removed.  Does this mean that this is
no longer appropriate?  If so, consider mentioning this user-behavior
change in NEWS: what was effectively a user option (per the doc
suggestion) is no longer such.
 
Or if this is still intended as something for a user's init file, then
please make it a proper user option (defcustom).

In GNU Emacs 24.0.92.1 (i386-mingw-nt5.1.2600) of 2011-11-30 on MARVIN
 Windowing system distributor `Microsoft Corp.', version 5.1.2600
 configured using `configure --with-gcc (4.6) --no-opt --cflags
 -ID:/devel/emacs/libs/libXpm-3.5.8/include
 -ID:/devel/emacs/libs/libXpm-3.5.8/src
 -ID:/devel/emacs/libs/libpng-dev_1.4.3-1/include
 -ID:/devel/emacs/libs/zlib-dev_1.2.5-2/include
 -ID:/devel/emacs/libs/giflib-4.1.4-1/include
 -ID:/devel/emacs/libs/jpeg-6b-4/include
 -ID:/devel/emacs/libs/tiff-3.8.2-1/include
 -ID:/devel/emacs/libs/gnutls-2.10.1/include --ldflags
 -LD:/devel/emacs/libs/gnutls-2.10.1/lib'