bug#12282: 24.1.50; `font-lock-value-in-major-mode': doc string and parameter name

[Drew Adams]

The parameter should be renamed.  It is an alist associating major modes
with font-lock decoration levels OR it is such a level.
 
The current doc and parameter name give the impression that the argument
passed is typically an alist.  But in practice (existing code in Emacs
distribution, as well as existing 3rd-party code), it is more often a
decoration level (including `t' for max decoration) that is passed as
argument.
 
Just saying that the return value is ALIST if ALIST is not an alist,
while certainly correct, is being less clear than we could be.

In GNU Emacs 24.1.50.1 (i386-mingw-nt5.1.2600)
 of 2012-08-13 on MARVIN
Bzr revision: 109584 monnier@iro.umontreal.ca-20120813141247-76irjqslrfncn30u
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (4.6) --no-opt --enable-checking --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-3.0.9/include
 -ID:/devel/emacs/libs/libiconv-1.13.1-1-dev/include
 -ID:/devel/emacs/libs/libxml2-2.7.8/include/libxml2'