Document the /logout api

We have one, and it's useful, so we really ought to document it

api/client-server/logout.yaml

@@ -0,0 +1,32 @@
+swagger: '2.0'
+info:
+  title: "Matrix Client-Server Registration and Login API"
+  version: "1.0.0"
+host: localhost:8008
+schemes:
+  - https
+  - http
+basePath: /_matrix/client/%CLIENT_MAJOR_VERSION%
+consumes:
+  - application/json
+produces:
+  - application/json
+securityDefinitions:
+  $ref: definitions/security.yaml
+paths:
+  "/logout":
+    post:
+      summary: Invalidates a user access token
+      description: |-
+        Invalidates an existing access token, so that it can no longer be used for
+        authorization.
+      security:
+        - accessToken: []
+      responses:
+        200:
+          description: The access token used in the request was succesfully invalidated.
+          schema:
+            type: object
+            properties: []
+      tags:
+        - Session management