Document behavior of generating destination path when copying

[phinate]

Documents #1497.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.27%. Comparing base (5f7dc40) to head (55aa924). Report is 31 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1498      +/-   ##
==========================================
- Coverage   97.46%   97.27%   -0.20%     
==========================================
  Files          48       48              
  Lines        4537     4543       +6     
==========================================
- Hits         4422     4419       -3     
- Misses        115      124       +9     

Flag	Coverage Δ
unittests	97.27% <ø> (-0.20%)	:arrow_down:

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[yajo]

There are formatting issues. Please run poetry run poe lint and push the new changes.