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

[Eli Zaretskii]

> From: "Drew Adams" <drew.adams@oracle.com>
> Date: Sat, 10 Nov 2012 08:46:24 -0800
> 
> It should say that the string can be any string, and that the function
> returns non-nil for #1 or #2, and nil otherwise:
>  
> 1. a defined (recognized) color name
> 2. a color RGB code, with prefix `#'
>  
> The doc should not say anything about the particular strings
> "unspecified-fg" or "unspecified-bg": there is nothing special about
> them.  The function returns nil for ANY string other than #1 or #2.

They _are_ special, because they are recognized color names.