Cleaning up Extension Documentation and ClassGenerator Notes

[mavaylon1]

Clean up and synchronize documentation regarding extensions and class generation.

Checklist

• [x] Did you update CHANGELOG.md with your changes?
• [x] Does the PR clearly describe the problem and the solution?
• [x] Have you reviewed our Contributing Guide?
• [ ] Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 91.68%. Comparing base (8c80a8b) to head (1047767). :warning: Report is 12 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1268   +/-   ##
=======================================
  Coverage   91.68%   91.68%           
=======================================
  Files          42       42           
  Lines        9599     9600    +1     
  Branches     1933     1934    +1     
=======================================
+ Hits         8801     8802    +1     
  Misses        519      519           
  Partials      279      279           

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[rly]

Thanks @mavaylon1 !