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.
> They're just the things that Homebrew installs.

How would someone know that? For what it's worth I think all those things are confusing/jargony and have misunderstood several of them in the past.