reword doscstrings and user guide psi

[solegalli]

closes #858
closes #859

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.04%. Comparing base (1fc4a43) to head (c9c3385).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #870   +/-   ##
=======================================
  Coverage   98.04%   98.04%           
=======================================
  Files         111      111           
  Lines        4653     4653           
  Branches      742      742           
=======================================
  Hits         4562     4562           
  Misses         56       56           
  Partials       35       35           

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