async_cluster: improve docs

[utkarshgupta137]

Pull Request check-list

• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

Description of change

• move response_callbacks kwarg inside init to hide it from docs
• use :member-order: bysource to group similar commands together

[utkarshgupta137]

@dvora-h The documentation for async cluster is missing for existing versions: https://redis-py.readthedocs.io/en/v4.3.0/connections.html#async-cluster-client

[codecov-commenter]

Codecov Report

Merging #2208 (19e5d32) into master (a2365d1) will decrease coverage by 0.00%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master    #2208      +/-   ##
==========================================
- Coverage   91.84%   91.84%   -0.01%     
==========================================
  Files         108      108              
  Lines       27691    27690       -1     
==========================================
- Hits        25433    25432       -1     
  Misses       2258     2258              

Impacted Files	Coverage Δ
redis/asyncio/cluster.py	89.91% <93.33%> (-0.02%)	⬇️

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