docs(providers): info about the B2 S3-compatible endpoint

[DarkArc]

This expands the note about using B2 storage as S3-compatible storage by providing some additional context on where to get the endpoint value.

I've also changed the redundant fully qualified "Backblaze B2" to "B2" in the note.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.17%. Comparing base (cb455c6) to head (16a0712).
Report is 139 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3860      +/-   ##
==========================================
+ Coverage   75.86%   77.17%   +1.31%     
==========================================
  Files         470      479       +9     
  Lines       37301    28756    -8545     
==========================================
- Hits        28299    22193    -6106     
+ Misses       7071     4666    -2405     
+ Partials     1931     1897      -34     

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