Improve IBD documentation and IdentitySegments docs

[benjeffery]

Extracted from #3329

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.72%. Comparing base (5ffcf6f) to head (208cdda).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3330   +/-   ##
=======================================
  Coverage   89.72%   89.72%           
=======================================
  Files          29       29           
  Lines       31181    31181           
  Branches     5720     5720           
=======================================
  Hits        27976    27976           
  Misses       1796     1796           
  Partials     1409     1409           

Flag	Coverage Δ
c-tests	86.71% <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	33.55% <ø> (ø)
python-tests-numpy1	50.10% <ø> (ø)

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Files with missing lines	Coverage Δ
python/tskit/tables.py	99.77% <ø> (ø)
python/tskit/trees.py	98.89% <ø> (ø)

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