Document that the filter doesn't "know" any CSS/xcolor predefined color names

[bpj]

Document that the filter doesn't "know" any CSS/xcolor predefined color names!

Rather the filter trusts the user to know and use valid color names. This is kind of intentional since users may define custom color names in their LaTeX (preamble) source. It is also kind of necessary since otherwise the filter would have to include a huge table of all the CSS color names and check color names against it in HTML mode. However it is important to be aware because LaTeX (xcolor) will choke on invalid color names while CSS will just ignore them.