[RLlib] Move __grouping_doc_end__

[bveeramani]

Why are these changes needed?

These changes are needed for two reasons.

__grouping_doc_end__ is in the wrong place
If you look at the part of the Ray documentation where the tag is referenced, you'll read

You can use the MultiAgentEnv.with_agent_groups() method to define these groups:

However, if you look at the code snippet below, you'll see the implementation of to_base_env in addition to the implementation of with_agent_groups.

To remove to_base_env from the code snippet, we need to move __grouping_doc__end__.

Black cannot format multi_agent_env.py
For some reason, Black errors while formatting multi_agent_env.py. However, if we move __grouping_doc_end__ up, the issue is resolved.

Related issue number

Closes #21314. Also see #21311.

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[pcmoritz]

Thanks, this looks great! I also checked that it indeed fixes the documentation :)