Check box group: documented anatomy doesn't match what actually gets rendered

[francisfuzz]

Summary

Currently, there's not any way to configure the "position" of the Caption to match what's in its documented Anatomy. 😅

The Primer Docs' anatomy of a checkbox group orders the following from top to bottom:

• Label
• Caption
• Options
• Validation message

However, peeking at the source for a check box group, it would appear what's actually rendered is in a different order (again, from top to bottom):

• Label
• Options
• Validation Message
• Caption

I think it would be neat to reflect parity in both the documentation and implementation! 🙇

[francisfuzz]

#3072 is targeted to address this 🎯