docs: improve help text for --defaultThemeVariants CLI flag

[PKulkoRaccoonGang]

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 94.39%. Comparing base (dfc69e2) to head (4c8a4b3).
⚠️ Report is 8 commits behind head on release-23.x.

Additional details and impacted files

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

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