Clarify signatures upload endpoint (#1495)

* clarify signatures upload endpoint

* add changelog

changelogs/client_server/newsfragments/1495.clarification

@@ -0,0 +1 @@
+Clarify parts of the end-to-end encryption sections.

data/api/client-server/cross_signing.yaml

@@ -149,8 +149,12 @@ paths:
       x-addedInMatrixVersion: "1.1"
       summary: Upload cross-signing signatures.
       description: |-
-        Publishes cross-signing signatures for the user.  The request body is a
-        map from user ID to key ID to signed JSON object.
+        Publishes cross-signing signatures for the user.
+
+        The request body is a map from user ID to key ID to signed JSON object.
+        The signed JSON object must match the key previously uploaded or
+        retrieved for the given key ID, with the exception of the `signatures`
+        property, which contains the new signature(s) to add.
       operationId: uploadCrossSigningSignatures
       security:
         - accessToken: []
@@ -158,7 +162,8 @@ paths:
         - in: body
           name: signatures
           description: |-
-            The signatures to be published.
+            A map from user ID to key ID to signed JSON objects containing the
+            signatures to be published.
           required: true
           schema:
             type: object