Rename responsive utilities to visibility utilities

[JacobLett]

I think the naming is a little confusing and could be clearer on their purpose by renaming it to be more literal on what they do. Someone could be searching the docs thinking, "how do I hide this div on mobile? Ahh visibility utilities."

[bkdotcom]

another alternative name:
breakpoint utilities?

[mdo]

Got a doozy of a PR for you at #22113 to address this and some other things I've been noodling on around these utilities :). Reviews and general feedback would be awesome!