Return a managed document if-present, else create one pointing at disk.

        See https://github.com/Microsoft/language-server-protocol/issues/177

def get_document(self, doc_uri):
        """Return a managed document if-present, else create one pointing at disk.

        See https://github.com/Microsoft/language-server-protocol/issues/177
        """
        return self._docs.get(doc_uri) or self._create_document(doc_uri)