doc: document that labels must have a dot in them to have an effect...
doc: document that labels must have a dot in them to have an effect
I noticed that the `hg topics` template has a bare `topic` label with
no dot, and that makes it useless, as such a label will never receive
any effect by the colour extension.
This dot has been required for a long time, at least since 2011, but
we never formally documented it!
