Fix warnings and code styling in the doc

There was a blank line missing that was generating a warning when
building the doc. Also, a removed function from cli.py was left in api.rst
(without_appcontext specifically) so this was also generating warnings.

Also corrects the format for a code block and adds a link to Werkzeug's Headers
class.

flask/testing.py

@@ -54,6 +54,8 @@ class FlaskClient(Client):
         the test client uses.  Once the with block is left the session is
         stored back.
 
+        ::
+
             with client.session_transaction() as session:
                 session['value'] = 42