Make it clearer what the rummager rake tasks do

It wasn't clear that these tasks index more than just "documents". In
fact they either index detailed guides, or everything else that is
searchable.
Rename misleading methods

At some point in the distant past, Whitehall.government_edition_classes
did actually return only Edition classes. Now, it returns all classes
that are searchable and should be indexed in the "government" index,
i.e. all searchable classes except DetailedGuide. I've renamed these
methods to avoid confusion in the future.
Merge pull request #1656 from alphagov/rewrite-code-for-clarity

Rewrite code for clarity