added docstrings to theories in the stdlib

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.52%. Comparing base (bfb008e) to head (6b60c2e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #143 +/- ## ======================================= Coverage 94.52% 94.52% ======================================= Files 38 38 Lines 2082 2082 ======================================= Hits 1968 1968 Misses 114 114 ```

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[quffaro]

Base.repr renders the @op macro in ThPreorder as a LineNumberNode