fix docs for AWS_... settings. A follow-up to GH-2609.

[kmike]

• labels were after setting description, while they should have been before them;
• BOT_NAME label was misplaced;
• new options from S3FilesStore support for other S3 providers (botocore options) #2609 only work with botocore;
• document AWS_REGION_NAME introduced in S3FilesStore support for other S3 providers (botocore options) #2609.

[codecov]

Codecov Report

Merging #3548 into master will not change coverage.
The diff coverage is n/a.

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  Coverage   84.38%   84.38%           
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  Files         167      167           
  Lines        9376     9376           
  Branches     1392     1392           
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  Hits         7912     7912           
  Misses       1206     1206           
  Partials      258      258