V4 dev updates and backports

TODO:

• Simplify/improve the newly added scrollspy test cases
• Tweak the newly added validation workaround examples if needed
• Do we need to port the workaround to master too?
• Review the wording changes
• Make a final rebase before merge

Preview:

• https://deploy-preview-30768--twbs-bootstrap.netlify.app/
• https://deploy-preview-30768--twbs-bootstrap.netlify.app/docs/4.4/components/forms/#input-group-validation-workaround