Add admin api for sending server_notices (#5121)

docs/admin_api/server_notices.md

@@ -0,0 +1,48 @@
+# Server Notices
+
+The API to send notices is as follows:
+
+```
+POST /_synapse/admin/v1/send_server_notice
+```
+
+or:
+
+```
+PUT /_synapse/admin/v1/send_server_notice/{txnId}
+```
+
+You will need to authenticate with an access token for an admin user.
+
+When using the `PUT` form, retransmissions with the same transaction ID will be
+ignored in the same way as with `PUT
+/_matrix/client/r0/rooms/{roomId}/send/{eventType}/{txnId}`.
+
+The request body should look something like the following:
+
+```json
+{
+    "user_id": "@target_user:server_name",
+    "content": {
+        "msgtype": "m.text",
+        "body": "This is my message"
+    }
+}
+```
+
+You can optionally include the following additional parameters:
+
+* `type`: the type of event. Defaults to `m.room.message`.
+* `state_key`: Setting this will result in a state event being sent.
+
+
+Once the notice has been sent, the APU will return the following response:
+
+```json
+{
+    "event_id": "<event_id>"
+}
+```
+
+Note that server notices must be enabled in `homeserver.yaml` before this API
+can be used. See [server_notices.md](../server_notices.md) for more information.