Unify notation in the quantum_info documentation

[renatomello]

This PR unifies notation between the quantum info module and its upcoming paper

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 99.51%. Comparing base (1710c99) to head (6f80180).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1511   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files          78       78           
  Lines       14098    14098           
=======================================
  Hits        14029    14029           
  Misses         69       69           

Flag	Coverage Δ
unittests	99.51% <100.00%> (ø)

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