Code review comment for lp:~wgrant/launchpad/bug-708999-ff-docs

After much discussion on IRC, I went with a more thorough revamp. I standardised the boolean type to empty/non-empty (which most of the flags already respected, despite their docs), and documented the types. I also expanded the descriptions, documented soyuz.derived-distros-ui.enabled, and added a link to further documentation on the dev wiki.

The new pages can be seen at <http://people.canonical.com/~wgrant/launchpad/feature-flag-docs/>.

Only one flag will require changes on production: "memcache pageid:ProjectGroup:+milestones 13 disabled" needs an s/disabled//.