Docs: tweak icon link and callouts (#37923)

Docs: tweak icon link and callouts (#37923)

* Tweak callout for icon link

- no need to talk about `alt` for `<img>` since these don't use `<img>`
- make start of sentence more readable

* Tweak language of dropdown warning callout

* Tweak popover warning callout

* Tweak rtl warning callouts

* Tweak color warning callout

* Add/move accessibility callout for color to relevant pages

* Update site/content/docs/5.3/helpers/icon-link.md

* Update site/layouts/partials/callouts/warning-color-assistive-technologies.md

Co-authored-by: Mark Otto <markd.otto@gmail.com>