`.form-text` follow-up

Created by: MartijnCuppens

Follow up on https://github.com/twbs/bootstrap/pull/30565

• We use <small> in our docs, which might not always be correct, the font size is not smaller per se.
• We still use text-muted in the horizontal form here: https://twbs-bootstrap.netlify.com/docs/4.3/forms/overview/#form-text
• We should share properties between .form-text, .valid-feedback and .invalid-feedback

I think we better remove display: block from .form-text and replace the <small>s with <div>s and use a <span> in the horizontal form.