docs: improve help text for --defaultThemeVariants CLI flag

[peterkulko]

Description

Rewrites the --defaultThemeVariants help text for the build-scss CLI command to explain what the flag actually does, why you'd use it, and what happens to non-default variants

Issue: #3953

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.40%. Comparing base (dfc69e2) to head (64d3362).
⚠️ Report is 37 commits behind head on release-23.x.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-23.x    #4154      +/-   ##
================================================
+ Coverage         94.39%   94.40%   +0.01%     
================================================
  Files               242      242              
  Lines              4296     4306      +10     
  Branches           1020      981      -39     
================================================
+ Hits               4055     4065      +10     
  Misses              237      237              
  Partials              4        4              

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[brian-smith-tcril]

This is great! One tiny comment with a question/suggestion.

[brian-smith-tcril]

It's not clear what

Variants not listed as defaults are still built and available but must be explicitly selected by the consuming application.
means here. Specifically what "explicitly selected" means.

I think the description is already strong without that sentence. How would you feel about just not including it?

[peterkulko]

Agreed, removed

[openedx-semantic-release-bot]

🎉 This PR is included in version 23.21.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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