Document the server_name query parameter on /join/{roomIdOrAlias}

Fixes https://github.com/matrix-org/matrix-doc/issues/904

api/client-server/joining.yaml

@@ -143,6 +143,16 @@ paths:
           description: The room identifier or alias to join.
           required: true
           x-example: "#monkeys:matrix.org"
+        - in: query
+          type: array
+          items:
+            type: string
+          explode: true
+          name: server_name
+          description: |-
+            The servers to attempt to join the room through. One of the servers
+            must be participating in the room.
+          x-example: ["matrix.org", "elsewhere.ca"]
         - in: body
           name: third_party_signed
           schema: