Add SigV4 note to generate_presigned_post documentation

[adev-code]

Issue

https://github.com/boto/boto3/issues/4351 Title: (short issue description)generate_presigned_post documentation should provide a link to S3 doc explaining regional default variation in response #4351

Summary

This PR adds a documentation note to generate_presigned_post in signers.py, explaining the difference between Signature Version 2 (SigV2) and Signature Version 4 (SigV4). As suggested on Issue 4351, it would be feasible if the information from the S3 API Documentation https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-authenticating-requests.html would be included to the Boto3 Doc: https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/s3/client/generate_presigned_post.html#generate-presigned-post

Changes

• Added a .. note:: section in signers.py docstring.
• Included a link to AWS's official SigV4 documentation and its note.

Testing

• Ran make html in docs/ to verify the doc renders correctly.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.97%. Comparing base (6300f6b) to head (303c516). Report is 450 commits behind head on develop.

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3421      +/-   ##
===========================================
+ Coverage    92.94%   92.97%   +0.03%     
===========================================
  Files           66       67       +1     
  Lines        14956    15054      +98     
===========================================
+ Hits         13901    13997      +96     
- Misses        1055     1057       +2     

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[SamRemis]

I'd rather see us make a more informative note that clarifies which signature version will be used by default with botocore, and why customers may want to change it, then link to the docs page where this was copied from.

Just copy/pasting the note from another AWS docs site could create conflicting information if the other page ever updates that note.