bug#12853: 24.2.50; doc of `color-defined-p'

[Drew Adams]

> So these two functions do not support 1-to-1 correspondence of color
> names, as you seem to assume.

I do not assume anything about these functions.

> That would remove information that's important for someone who writes
> code which manipulates color names at a low level where these
> distinctions are important.  It was important for me when I did that,
> and so I would object to removing this.

I'm not asking you to remove anything.  I'm asking that the doc be corrected and
made more clear and complete.

If you feel it is important to mention those two noncolors here, please do so.
And consider mentioning also why that is important: why this function deals with
such exceptions.

If it is not appropriate to mention `defined-colors' here then, by all means, do
not do so.  I was hunting for some definition of "defined color" that might be
pertinent here, but apparently that was a false route.

In any case, please make clear here just what is meant by a "defined color" in
this context, and how that definition relates to those particular noncolor
defined-color exceptions.

This is all the more important if, as seems to be the case, "defined color" here
is something not at all obvious, with kludgy exceptions.

IOW, if the meaning of "defined color" here were straightforward and we could
simply point to some other function (e.g. `defined-colors'), then doing that
would be appropriate and sufficient to make clear what that term means here.
Since the meaning is NOT obvious or straightforward, it needs to be provided
(explained).