[MRG+1] Improve docs of S3 Storage to make Python version more explicit

[rennerocha]

The documentation of the S3 storage backend (https://doc.scrapy.org/en/latest/topics/feed-exports.html#s3) suggest the use of the libraries botocore (https://github.com/boto/botocore) or boto (https://github.com/boto/boto). However, boto library will work only if the user is using Python 2 (as it is correctly explained here https://doc.scrapy.org/en/latest/topics/feed-exports.html#storages).

Besides the fact of this information is there, I believe that it would be better for the user reading the documentation to have this note also inside the specific section of S3 storage backend.

When issue #1866 is resolved, this could be removed anyway.

[codecov]

Codecov Report

Merging #3294 into master will not change coverage.
The diff coverage is n/a.

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