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 99.72%. Comparing base (05b57e9) to head (1a2552b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1511   +/-   ##
=======================================
  Coverage   99.72%   99.72%           
=======================================
  Files          80       80           
  Lines       11781    11781           
=======================================
  Hits        11749    11749           
  Misses         32       32           

Flag	Coverage Δ
unittests	99.72% <ø> (ø)

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