Fix CI, update docstrings

[mattwthompson]

CI has been failing for a few days because the smirnoff99Frosst line is no longer installed by default¹. This fixes that, updates some docstrings, and tidies up some formatting I didn't like.

Footnotes

1. https://github.com/conda-forge/openff-toolkit-feedstock/pull/113 ↩

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.14%. Comparing base (48a0b54) to head (6ca9618).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #408   +/-   ##
=======================================
  Coverage   84.14%   84.14%           
=======================================
  Files           5        5           
  Lines         776      776           
=======================================
  Hits          653      653           
  Misses        123      123           

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