Update RTM docs to reflect `rtm.start` deprecation for certain apps

[srajiang]

Current tools docs 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

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[ryan-aldaz]

Hi! I would let to help out. I'm assuming this link explain the situation here
How should the info about be add?

[srajiang]

Hey @ryan-aldaz - nice, we love contributions! Looks like you'll want to update the real_time_messaging doc source

Here are some helpful instructions with info on how we generate docs source.