Document Redis(single_connection_client)

[povilasb]

Pull Request check-list

Please make sure to review and check all of these items:

• [X] Does $ tox pass with this change (including linting)?
• [X] Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• [X] Is the new or changed code fully tested?
• [X] Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• [X] Is there an example added to the examples folder (if applicable)?
• [ ] Was the change added to CHANGES file? WON'T DO: too insignificant.

Description of change

I spent a significant portion of time debugging concurrency issues not knowing that Redis is not thread safe with single_connection_client=True. Found this comment on GitHub issues: https://github.com/redis/redis-py/issues/1426#issuecomment-740298873 Hopefully will save the trouble for others :)

[codecov-commenter]

Codecov Report

Merging #2327 (a8a1166) into master (2cea637) will decrease coverage by 0.02%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2327      +/-   ##
==========================================
- Coverage   92.21%   92.18%   -0.03%     
==========================================
  Files         111      111              
  Lines       28751    28751              
==========================================
- Hits        26513    26505       -8     
- Misses       2238     2246       +8     

Impacted Files	Coverage Δ
redis/client.py	89.06% <ø> (ø)
tests/conftest.py	84.94% <0.00%> (-0.78%)	:arrow_down:
tests/test_asyncio/test_search.py	98.43% <0.00%> (-0.32%)	:arrow_down:
tests/test_cluster.py	96.85% <0.00%> (-0.24%)	:arrow_down:

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