text: Title: Update RTM docs to reflect `rtm.start` deprecation for certain apps
  Body: [Current tools docs](https://slack.dev/python-slack-sdk/real_time_messaging.html) could be updated to reflect changelog entry regard `rtm.start` method deprecation for apps created after Nov 30th, 2021. 
  
  ### The page URLs
  
  - https://slack.dev/python-slack-sdk/real_time_messaging.html
  
  ### Requirements
  
  Please read the [Contributing guidelines](https://github.com/slackapi/python-slack-sdk/blob/main/.github/contributing.md) and [Code of Conduct](https://slackhq.github.io/code-of-conduct) before creating this issue or pull request. By submitting, you are agreeing to those rules.
  