bug#8049: 24.0.50; doc of `list-colors-sort'

[Drew Adams]

The doc should explain what is meant by RGB and HSV color distance here.  There
is not only one possible definition of color distance, and many users will not
have a feel for this or prior knowledge about it.

There is nothing in either Emacs manual about this, so the doc string needs to
cover it.
 
In GNU Emacs 24.0.50.1 (i386-mingw-nt5.1.2600)
 of 2011-02-14 on 3249CTO
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.4) --no-opt --cflags
-Ic:/imagesupport/include'