color: update main documentation
Now that the feature no longer lives in the extension, we document it in the
help of the core config. This include the new 'ui.color' option introduced in
the previous changesets.
As a result the color extensions can now be deprecated.
This is a documentation patch only; color is still disabled by default.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).