bug#7774: 24.0.50; `frame-background-mode'

[Drew Adams]

emacs -Q
 
There needs to be some explanation of `frame-background-mode' for users
and for Emacs-Lisp programmers.
 
First, the option is described briefly in the Elisp manual, but not in
the Emacs manual.  It is a _user_ option, and should be in the latter.
 
Second, the (elisp) entry does not say what happens if the value is
nil.  The doc string does say that if it is nil then Emacs will "examine
the brightness for you".  That explanation of nil is incomprehensible
and pretty useless for users, but it is especially useless for
programmers. 
 
I had some code that tried to DTRT for a given color based on the
`frame-background-mode'.  I was incorrectly assuming that the value was
set to `light' or `dark' automatically by Emacs ("examine the brightness
for you") if it was not already set (customized) by the user.
 
Now I guess I will need to call `frame-set-background-mode' if the value
is nil, to determine the proper value.  The doc should have helped here.
 

In GNU Emacs 24.0.50.1 (i386-mingw-nt5.1.2600)
 of 2010-12-20 on 3249CTO
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.4) --no-opt --cflags
-Ic:/imagesupport/include'