improve documentation for session attributes

add test for session attributes

tests/test_basic.py

@@ -221,12 +221,21 @@ def test_endpoint_decorator(app, client):
 def test_session(app, client):
     @app.route('/set', methods=['POST'])
     def set():
+        assert not flask.session.accessed
+        assert not flask.session.modified
         flask.session['value'] = flask.request.form['value']
+        assert flask.session.accessed
+        assert flask.session.modified
         return 'value set'
 
     @app.route('/get')
     def get():
-        return flask.session['value']
+        assert not flask.session.accessed
+        assert not flask.session.modified
+        v = flask.session.get('value', 'None')
+        assert flask.session.accessed
+        assert not flask.session.modified
+        return v
 
     assert client.post('/set', data={'value': '42'}).data == b'value set'
     assert client.get('/get').data == b'42'