devbook.xsl has this (which looks like it was cut and pasted from the infra-status page): <meta name="description" content="Gentoo Infrastructure Status provides information on the status of the services the Gentoo Linux distribution offers to its developers and users." />
How about the following? Taken from the old project page and shortened a bit (to keep it below the 160 character limit of popular search engines). "The Gentoo Devmanual is a technical manual which covers topics such as writing ebuilds and eclasses, and policies that developers should be abiding by."
commit 5e21123c21c086132a52cc261bcd138e7614aae4 Author: Ulrich Müller <ulm@gentoo.org> Date: Tue Jun 2 18:58:28 2015 +0200 devbook.xsl: Fix description meta tag. Bug 550942.