DuplicateFreeList: Make documentation more precise

[TWiedemann]

Clarifies how the order of the entries in DuplicateFreeList(oldList) is determined.

This came up here, where it matters in which order some files are scanned: gap-packages/AutoDoc#382

Text for release notes

none.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.23%. Comparing base (b2ebc4f) to head (a34c47c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6625   +/-   ##
=======================================
  Coverage   79.23%   79.23%           
=======================================
  Files         683      683           
  Lines      295181   295185    +4     
  Branches     8647     8647           
=======================================
+ Hits       233892   233898    +6     
+ Misses      59471    59470    -1     
+ Partials     1818     1817    -1     

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