feat(Affordance): Add visibility notes

Purpose

In Unbox linked widgets (gitlab-org/gitlab!113820 - merged) we were initially proposing to only make the ability to remove a related item control (via a × remove button) available through :hover or :focus-within. However, after further discussion and exploration it was determined that this could prevent some users from either knowing the ability to remove existed, or to complete the task or removing related items impossible altogether. In this particular instance, there's no other place in the UI where such a task can be done, and for that reason it must be readily available for all users.

Scott O'Hara weighed in in a separate Slack team and had some good points worth capturing here (also captured in the linked MR above).

visually hiding controls, especially depending on the way they were hidden can still pose blockers for some people. e.g., how am i supposed to know to say “click delete button” if i’m using voice control software with no mouse or keyboard, if teh buttons aren’t there for me to see? similarly there’s no hover/focus (unless you’re using a bluetooth keyboard) for touch devices. so, guess one just has to randomly tap around the screen to see if a focus / simulated hover event fires to reveal the hidden controls

constant struggle between visual ‘clutter’ and accessibility for all.

different types of users need different things, and what works well for one user might be distracting / not what the other wants.