text: Title: Spelling of HTTP 429 error is inconsistent
  Body: Hello!
  
  I am implementing a Slack app, and I am trying to implement correct backoff handling for when Slack reports my app has exceeded rate limits.
  
  The [example](https://github.com/slackapi/python-slack-sdk/blob/main/docs-src-v2/basic_usage.rst#web-api-rate-limits) in the docs says you can notice that you are being rate limited with this snippet:
  ```python   
  try:
      response = send_slack_message(channel, message)
  except SlackApiError as e:
      if e.response["error"] == "ratelimited":
          ...
  ```
  
  The method in question is `chat.postMessage`. However, the documentation for the [errors](https://api.slack.com/methods/chat.postMessage#errors) of that method say that the rate limiting error is spelled `rate_limited` (with an underscore).
  
  I clicked around at random on a few web API methods on the API docs, and I do see a mixture of ratelimited and rate_limited spellings.
  The places _in code_ (i.e. not in the docs) all seem to be calling methods that spell the error "ratelimited" (`rtm.connect` and `apps.connections.open`), so the good news is that this mostly seems to be incorrect documentation?
  
  Maybe this is a complaint better lodged at the API itself. For my application, I'm just going to check the HTTP status code, which is what the [page on rate limits](https://api.slack.com/docs/rate-limits) recommends.
  
  I'd be happy to send a PR for the docs, but I'm wondering what you'd consider the best approach: should the docs check the HTTP status code instead of the error message? Should the docs suggest `rate_limited` when calling `chat.postMessage` (as in the snippet above)?
  Thanks!