make path → URL resolution clearer.

You can't append a string to an object (at least not with a canonical result), so we should make clear that we want to append to the `url` field of the server object (possibly after filling in the templates there).

The highlighting of "appended" makes it clearer that there is no relative URL resolution as mentioned in https://github.com/OAI/OpenAPI-Specification/issues/843#issuecomment-261575257.