api-docs: Add when moving messages fields/parameters were added.

Adds API changelog feature level 1 and associated Changes notes
for when the `stream_id` parameter in the `PATCH /messages/message_id`
was added, and for when the `prev_stream` field was added to edit
history information for messages.

We're adding these to the Zulip 3.0 feature level 1 because
commit 843345dfee that introduced this field and this parameter
to the server / backend code was merged before the commit that added
the API feature level tracking, commit e3b90a5ec8, at level 1.

api_docs/changelog.md

@@ -1461,6 +1461,15 @@ No changes; feature level used for Zulip 3.0 release.
 
 **Feature level 1**:
 
+* [`PATCH /messages/{message_id}`](/api/update-message): Added the
+  `stream_id` parameter to support moving messages between streams.
+* [`GET /messages`](/api/get-messages), [`GET /events`](/api/get-events):
+  Added `prev_stream` as a potential property of the `edit_history` object
+  within message objects to indicate when a message was moved to another
+  stream.
+* [`GET messages/{message_id}/history`](/api/get-message-history):
+  `prev_stream` is present in `snapshot` objects within `message_history`
+  object when a message was moved to another stream.
 * [`GET /server_settings`](/api/get-server-settings): Added
   `zulip_feature_level`, which can be used by clients to detect which
   of the features described in this changelog are supported.

zerver/openapi/zulip.yaml

@@ -6004,6 +6004,8 @@ paths:
 
                                 The ID of the stream containing the message immediately
                                 prior to this edit event.
+
+                                **Changes**: New in Zulip 3.0 (feature level 1).
                             content:
                               type: string
                               description: |
@@ -7054,6 +7056,8 @@ paths:
             Note that a message's content and stream cannot be changed at the
             same time, so sending both `content` and `stream_id` parameters will
             throw an error.
+
+            **Changes**: New in Zulip 3.0 (feature level 1).
           schema:
             type: integer
           example: 43
@@ -17490,6 +17494,8 @@ components:
 
                   The stream ID of the message immediately prior to this
                   edit event.
+
+                  **Changes**: New in Zulip 3.0 (feature level 1).
               prev_topic:
                 type: string
                 description: |