Add documentation for enums created via the functional API

[Torvaney]

This PR updates the documentation to include discussion of enumerating choices with enum.Enums created with the functional API. This is useful when creating choices dynamically, for example logging.

See further discussion of enums and the functional API in https://github.com/tiangolo/typer/issues/389

[github-actions[bot]]

📝 Docs preview for commit e3f2d03773072e99d2f2ceab66d7fa4a22c2edd6 at: https://6283788fce83e3109147d75c--typertiangolo.netlify.app

[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (1e43c6b) to head (e3f2d03). Report is 126 commits behind head on master.

:exclamation: Current head e3f2d03 differs from pull request most recent head 4fe60e7. Consider uploading reports for the commit 4fe60e7 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #398      +/-   ##
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+ Coverage   96.24%   100.00%   +3.75%     
===========================================
  Files         280       252      -28     
  Lines        5942      5287     -655     
===========================================
- Hits         5719      5287     -432     
+ Misses        223         0     -223     

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[github-actions[bot]]

📝 Docs preview for commit 4fe60e7376eaebf028e61e7b567692fc48c4deea at: https://639cea4bf6fa040349687d3f--typertiangolo.netlify.app