message: docs: document FLASK_APP_MUTATOR
diff: @@ -201,6 +201,25 @@ CUSTOM_SECURITY_MANAGER = CustomSsoSecurityManager
  ]
  ```
  +### Flask app Configuration Hook
  +
  +`FLASK_APP_MUTATOR` is a configuration function that can be provided in your environment, receives
  +the app object and can alter it in any way. For example, add `FLASK_APP_MUTATOR` into your
  +`superset_config.py` to setup session cookie expiration time to 24 hours:
  +
  +```
  +def make_session_permanent():
  + '''
  + Enable maxAge for the cookie 'session'
  + '''
  + session.permanent = True
  +
  +# Set up max age of session to 24 hours
  +PERMANENT_SESSION_LIFETIME = timedelta(hours=24)
  +def FLASK_APP_MUTATOR(app: Flask) -> None:
  + app.before_request_funcs.setdefault(None, []).append(make_session_permanent)
  +```
  +
  ### Feature Flags
  To support a diverse set of users, Superset has some features that are not enabled by default. For
  