Don't use keg-only and pouring jargon.

This is unnecessarily confusing to end-users and the "fun" naming doesn't really communicate anything. @mistydemeo and I talked about this.

CC @adamv @mistydemeo @jacknagel @tdsmith @asparagui for thoughts and review.
"name is not linked into /usr/local" doesn't communicate the reason that we need `--force` (If `brew unlink` something, it is not linked into /usr/local, but I don't necessarily need `--force` to link it). I'm not sure how to phrase it without saying "keg-only" though.