chore: add clarifying comment

Co-authored-by: Carson Sievert <[email protected]>

Author: gadenbuie

shiny/www/py-shiny/brand/_brand-yml.scss

@@ -24,7 +24,7 @@
 
 // Sass variables from `brand` will be inserted (above) here in this order:
 // * brand.color.palette
-// * brand.defaults
+// * brand.defaults (Brand-defined Bootstrap defaults)
 // * brand.color
 // * brand.typography