Official descriptions rerun

[relgit]

This is a rerun of the scripts that created #626, off of the current state of the repository. It is meant to replace both #626 and #645, which have become full of unrelated commits.

To recap, in this PR:

• System manifest preference key descriptions updated with official texts where available, except for root descriptions where the official ones are empty
• Common preference keys in all manifests as well as in the manifest template updated with official descriptions
• Reference description tags removed from updated preference keys

[relgit]

I think that this PR has become too stale to merge at this point. If there's still interest in updating to the official descriptions, I'm happy to generate a new one after the current OS beta period.