Add changelog entry for /logout/all

Signed-off-by: Travis Ralston <travpc@gmail.com>

changelogs/client_server.rst

@@ -48,6 +48,8 @@ Unreleased changes
     (`#1142 <https://github.com/matrix-org/matrix-doc/pull/1142>`_).
   - Add the ``/register/available`` endpoint for username availability
     (`#1151 <https://github.com/matrix-org/matrix-doc/pull/1151>`_).
+  - Document ``/logout/all`` endpoint
+    (`#1263 <https://github.com/matrix-org/matrix-doc/pull/1263>`_).
 
 r0.3.0
 ======