Update Gentoo installation documentation now that we're in the portage tree proper

We're officially a first-class Gentoo citizen now, which is very exciting.  Many thanks to @gregkh for helping us get here.

I started just adapting sections of language in this document, and realized several bits needed to just be rewritten entirely to be more clear.
Merge pull request #2001 from tianon/gentoo-tree

Update Gentoo installation documentation now that we're in the portage tree proper