color: add basic documentation to 'debugcolor'

This does not hurt.

hgext/color.py

@@ -538,6 +538,7 b' def extsetup(ui):'
 
 @command('debugcolor', [], 'hg debugcolor')
 def debugcolor(ui, repo, **opts):
+    """show available colors and effects"""
     global _styles
     _styles = {}
     for effect in _effects.keys():