Add link to MSC in changelog entries.

changelogs/client_server/newsfragments/2536.feature

@@ -1 +1 @@
-Add cross-signing properties to the response of ``POST /keys/query``
+Add cross-signing properties to the response of ``POST /keys/query`` per [MSC1756](https://github.com/matrix-org/matrix-doc/pull/1756).

changelogs/client_server/newsfragments/2536.new

@@ -1 +1 @@
-``POST /keys/device_signing/upload`` and ``POST /keys/signatures/upload``
+Add ``POST /keys/device_signing/upload`` and ``POST /keys/signatures/upload`` per [MSC1756](https://github.com/matrix-org/matrix-doc/pull/1756).

changelogs/server_server/newsfragments/2536.feature

@@ -5,3 +5,5 @@ Add cross-signing:
 - The ``m.device_list_update`` EDU is sent when a device gets a new signature.
 - A new ``m.signing_key_update`` EDU is sent when a user's cross-signing keys
   are changed.
+
+per [MSC1756](https://github.com/matrix-org/matrix-doc/pull/1756).