Tidy the usage docs, including ancestral state specs

[hyanwong]

Also fixes #1024

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.41%. Comparing base (44827ed) to head (18b8467).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1025   +/-   ##
=======================================
  Coverage   93.41%   93.41%           
=======================================
  Files          18       18           
  Lines        6496     6496           
  Branches     1107     1107           
=======================================
  Hits         6068     6068           
  Misses        291      291           
  Partials      137      137           

Flag	Coverage Δ
C	93.41% <ø> (ø)
python	95.78% <ø> (ø)

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[jeromekelleher]

LGTM

[benjeffery]

Very nice, thanks! I found one teeny tiny nit.