docs(text): add module doc

[dsherret]

Reviewed (looks good) then added a module doc for the package.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.94%. Comparing base (0079ea0) to head (d7c4e5c).

❗ Current head d7c4e5c differs from pull request most recent head 07b4927

Please upload reports for the commit 07b4927 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4812      +/-   ##
==========================================
- Coverage   91.97%   91.94%   -0.03%     
==========================================
  Files         486      486              
  Lines       41461    41355     -106     
  Branches     5360     5321      -39     
==========================================
- Hits        38133    38024     -109     
- Misses       3270     3274       +4     
+ Partials       58       57       -1     

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[littledivy]

Can you also add text/mod.ts to _tools/check_docs.ts?

[lucacasonato]

LGTM