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.
Even if it's unintentionally clunky I think Mike's proposal is better than the status quo for newcomers, and general understanding as a whole. I wouldn't be unhappy to see ``` unlinked-by-default ``` merged if we can't agree on a better alternative. We can always come back to the name further down the line. I don't think many people spend enough time trying to understand Homebrew's gut terminology to notice/object if we tweak the phrasing beyond this initial PR.