Tidy extend_haplotypes docstring

[hyanwong]

And add doi. Fixes #3133

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.80%. Comparing base (dacbf38) to head (774bf07).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3315   +/-   ##
=======================================
  Coverage   89.80%   89.80%           
=======================================
  Files          29       29           
  Lines       31043    31043           
  Branches     5682     5682           
=======================================
  Hits        27877    27877           
  Misses       1779     1779           
  Partials     1387     1387           

Flag	Coverage Δ
c-tests	86.85% <ø> (ø)
lwt-tests	80.38% <ø> (ø)
python-c-tests	87.05% <ø> (ø)
python-tests	98.84% <ø> (ø)
python-tests-no-jit	33.60% <ø> (ø)
python-tests-numpy1	50.18% <ø> (ø)

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

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