createmarkers: allow to pass metadata for a marker only The `metadata` argument only allow to specify metadata for all new markers. We extension the format of the `relations` argument to support optional metadata argument. The first user of this should be the evolve extension who want to store parent information of pruned changeset in extra (until we make a second version of the format)

obsstore: update create docstring to point to the coder friendly function The `obsstore` class have a `create` method that create new obsolescence marker from node. There is another function in the same module `createmarkers`. This other function is higher level and automatically missing meta data (ultimately calling the first one) We add a new comment in the docstring of `obsstore.create` highlighting that people writing new code probably want to use the top level one.