Add docstrings to abstract Backend methods

[renatomello]

This PR adds docstrings to the abstract Backend methods that were introduced during the previous backend refactor.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.50%. Comparing base (1710c99) to head (270942a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1862      +/-   ##
==========================================
- Coverage   99.51%   99.50%   -0.01%     
==========================================
  Files          78       78              
  Lines       14098    14109      +11     
==========================================
+ Hits        14029    14039      +10     
- Misses         69       70       +1     

Flag	Coverage Δ
unittests	99.50% <100.00%> (-0.01%)	⬇️

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