Follow-up: format changelog, add licence (#10593)

Merged before approval; these comments from @clokep on that PR.

changelog.d/10593.bugfix

@@ -1 +1 @@
-Reject Client-Server /keys/query requests which provide device_ids incorrectly.
+Reject Client-Server `/keys/query` requests which provide `device_ids` incorrectly.

tests/rest/client/v2_alpha/test_keys.py

@@ -1,3 +1,17 @@
+#  Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License
+
 from http import HTTPStatus
 
 from synapse.api.errors import Codes