Add doc note re: mutations per site

[hyanwong]

See #3178 (reply in thread)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.62%. Comparing base (9d2ff8e) to head (a9de208).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3184      +/-   ##
==========================================
- Coverage   98.91%   89.62%   -9.29%     
==========================================
  Files          17       28      +11     
  Lines        7542    31969   +24427     
  Branches     1258     5873    +4615     
==========================================
+ Hits         7460    28653   +21193     
- Misses         33     1886    +1853     
- Partials       49     1430    +1381     

Flag	Coverage Δ
c-tests	86.66% <ø> (?)
lwt-tests	80.38% <ø> (?)
python-c-tests	88.18% <ø> (?)
python-tests	98.85% <ø> (ø)

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Files with missing lines	Coverage Δ
python/tskit/trees.py	98.88% <ø> (ø)

... and 11 files with indirect coverage changes

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