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.
``` private ``` feels alright to me. 

It feels natural enough for people to get, and is used outside of Homebrew, Public/Private APIs and such. If we think of it in the terms that everything in ``` /usr/local ``` is public, easily available to all and any software without needing to set up flags, specify directories, so on so forth, whereas ``` private ``` normally means having to temporarily play with the ``` $PATH ```, set flags, specify directories, etc to access, then it makes relatively natural sense.