Fix docstring error

[kris-brown]

Due to an external change, the result of @doc inside the @theory macro returns a Base.Docs.DocStr object rather than a Markdown.MD object. This broke some tests. This PR offers a partial fix. It is partial because the docstring one puts above @theory was copied in two places before: both the module and the GAT were given the same docstring before. However, I wasn't able to figure out how to get the fix to work for the GAT, though we do recover the same docstring as before at the module level.

Thus @doc ThCategory will work the same as before, but there is a regression insofar as ThCategory.Meta.theory will not have documentation. (Hence, two lines are commented out in the tests now)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.56%. Comparing base (a129cb4) to head (3edf6f4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #171      +/-   ##
==========================================
+ Coverage   93.85%   95.56%   +1.71%     
==========================================
  Files          38       38              
  Lines        2229     2232       +3     
==========================================
+ Hits         2092     2133      +41     
+ Misses        137       99      -38     

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