color: document that changeset phases have labels

It's very useful to be able to colourise csets according to their
phases. There was no indication anywhere in the docs that this is
possible.

We use e.g. `changeset.secret = ` instead of `changeset.secret
='none'`, because otherwise this is a BC: it would nullify the effects
given to log.changeset label that usually surrounds the
changeset.{phase} labels. Specifying the label without any effect
instead of 'none' is a true no-op change and purely documentation.

hgext/color.py

@@ -63,6 +63,11 b' effects may be overridden from your conf'
   diff.changed = white
   diff.trailingwhitespace = bold red_background
 
+  # Blank so it inherits the style of the surrounding label
+  changeset.public =
+  changeset.draft =
+  changeset.secret =
+
   resolve.unresolved = red bold
   resolve.resolved = green bold
 
@@ -275,6 +280,9 b' except ImportError:'
            'diff.hunk': 'magenta',
            'diff.inserted': 'green',
            'diff.trailingwhitespace': 'bold red_background',
+           'changeset.public' : '',
+           'changeset.draft' : '',
+           'changeset.secret' : '',
            'diffstat.deleted': 'red',
            'diffstat.inserted': 'green',
            'histedit.remaining': 'red bold',