Made explicit that duplicates are preserved.

[miselico]

Also changed the example such that there are duplicates. Added a comments to indicate what the content of the three input iterators is.

[phimuemue]

Good one, thank you.

Brings up the question which of the "equal" elements is preferred.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.80%. Comparing base (6814180) to head (81c45c7).
⚠️ Report is 185 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1094      +/-   ##
==========================================
- Coverage   94.38%   93.80%   -0.59%     
==========================================
  Files          48       51       +3     
  Lines        6665     6468     -197     
==========================================
- Hits         6291     6067     -224     
- Misses        374      401      +27     

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