api-docs: Remove sender_short_name from message event example.

Removes the `sender_short_name` from the example for the message
event in `/get-events`.

Also, to make this complete, adds Changes notes for the feature
level 26 changes that were made to the message objects returned
in the message events for `/get-events` and in the messages
array for the `/get-messages` response.

The field was originally removed from message objects in
commit b375581f58.

api_docs/changelog.md

@@ -1296,8 +1296,9 @@ releases.
 
 **Feature level 26**
 
-* [`GET /messages`](/api/get-messages): `sender_short_name` field is no
+* [`GET /messages`](/api/get-messages), [`GET /events`](/api/get-events):
-  longer included in return values for this endpoint.
+  The `sender_short_name` field is no longer included in message objects
+  returned by these endpoints.
 * [`GET /messages`](/api/get-messages) : Removed `short_name` field from
   `display_recipient` array objects.
 

zerver/openapi/zulip.yaml

@@ -846,6 +846,10 @@ paths:
                             - type: object
                               description: |
                                 Event type for messages.
+
+                                **Changes**: In Zulip 3.1 (feature level 26), the
+                                `sender_short_name` field was removed from message
+                                objects.
                               properties:
                                 id:
                                   $ref: "#/components/schemas/EventIdSchema"
@@ -888,7 +892,6 @@ paths:
                                       "reactions": [],
                                       "submessages": [],
                                       "sender_full_name": "King Hamlet",
-                                      "sender_short_name": "hamlet",
                                       "sender_email": "user10@zulip.testserver",
                                       "sender_realm_str": "zulip",
                                       "display_recipient": "Denmark",
@@ -5791,8 +5794,11 @@ paths:
                       messages:
                         type: array
                         description: |
-                          An array of `message` objects, each containing the following
+                          An array of `message` objects.
-                          fields:
+
+                          **Changes**: In Zulip 3.1 (feature level 26), the
+                          `sender_short_name` field was removed from message
+                          objects.
                         items:
                           allOf:
                             - $ref: "#/components/schemas/MessagesBase"