docs: Better indicate include options in sass setup.

[Kelketek]

Description

This change makes a light improvement to the docs on including additional modules in Sass.

Motivation & Context

I'm in the middle of an upgrade, and I just spent the last several hours trying to track down why my custom variables for forms weren't working only to realize that the list of includes on the Sass page was not intended to be comprehensive, but only examples of sass sheets one might want to import, and that one should go through the sass directory to find any components they may yet need.

The changes I'm proposing here would have saved me a few hours, so I imagine they'll help a few other folks, too :)

Type of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Refactoring (non-breaking change)
• [ ] Breaking change (fix or feature that would change existing functionality)

Checklist

• [x] I have read the contributing guidelines
• [x] My code follows the code style of the project (using npm run lint)
• [x] My change introduces changes to the documentation
• [x] I have updated the documentation accordingly
• [ ] I have added tests to cover my changes
• [x] All new and existing tests passed

Live previews

https://deploy-preview-41143--twbs-bootstrap.netlify.app/ (Pending CI)

[mdo]

That's fair, easy improvement here.