[Fixes #15928] Encourage use of icons with form validation states for colorblind users

Update text per @patrickhlauke's feedback

Update text per @patrickhlauke's feedback.

Fix stupid grammar mistake

Closes #15291 by merging it.

Document in the source code that the justified nav is meant for single line list items only. This should help prevent Issues like #15906 in the future.

Fixes #15106.
Closes #15910.

Added attributes rel and target.

Fixes #15882 — Use an ID as the example for Scrollspy so that it is more clear that this is for use with a single nav.

Update Creative Commons link to HTTPS.

Normalized the heading hierarchy in the documentation for the Button groups component.

Normalized the heading hierarchy in the documentation for the Dropdowns component.

Related to #15823, I would be interested in officially recommending a keyword such as `twbs-bootstrap` so that we can more easily showcase plugins and extensions for Bootstrap without requiring additional curation by the admins / collabs of TWBS

specificity

Add additional comment to docs

Fix capitalization

Update tag per @mdo's recommendation

Closes #15839 by merging a tweaked version of it.

Closes #15725 by merging a tweaked version of it.

[Fixes #15766] Need Clearer Affix plugin documentation

Remove trailing whitespace

Update the documentation

[skip sauce]

And add explanatory comments to the Carousel & Dashboard examples regarding holder.js

[skip sauce]