bug#3042: 23.0.92; display-visual class doc

The doc does not explain what the possible values mean. There are no
other occurrences of the various values (e.g. `static-color') in the
Elisp manual or other doc strings (that I can find) than what is said
about `display-visual-class'.
 
To use a function, it helps to know its possible values, but if you
have no idea what those values indicate or mean, then you really don't
know much about the function.
 
Grepping the Emacs sources doesn't really help here either. Looking at
the definition of `display-visual-class', you can at least learn
something (partial) about `static-color' and `static-gray', but that's
about it.
 

In GNU Emacs 23.0.92.1 (i386-mingw-nt5.1.2600)
 of 2009-03-30 on SOFT-MJASON
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (3.4)'