additional clarification about union behavior with populations

[petrelharp]

The existing documentation was NOT clear enough on this point.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.75%. Comparing base (99a9b34) to head (6cf0d1f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3264   +/-   ##
=======================================
  Coverage   89.75%   89.75%           
=======================================
  Files          29       29           
  Lines       32545    32545           
  Branches     5962     5962           
=======================================
  Hits        29212    29212           
  Misses       1890     1890           
  Partials     1443     1443           

Flag	Coverage Δ
c-tests	86.76% <ø> (ø)
lwt-tests	80.38% <ø> (ø)
python-c-tests	88.24% <ø> (ø)
python-tests	98.79% <ø> (ø)
python-tests-no-jit	33.02% <ø> (ø)
python-tests-numpy1	50.71% <ø> (ø)

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Files with missing lines	Coverage Δ
python/tskit/trees.py	98.86% <ø> (ø)

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