color: point to the config help in global help topic

We point out at the help of the config option for user who wants to learn more
about the possible config value.

The suggested command returns some other extra (color related) results, but this
is bug to fix outside of the freeze.

mercurial/help/color.txt

@@ -15,6 +15,8 b' To disable color use::'
   [ui]
   color = never
 
+See :hg:`help config.ui.color` for details.
+
 Mode
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