Add note about workaround about known issue with `asset_host`
BUG: Don't change `asset_providers` to nil on `Document`
Add TODO about raising error when trying to set asset_providers to nil

This is not implemented since it requires either a major (strict reading
of semver.org) or minor (fuzzy reasing of semver.org) version bump. I'll
queue up a few more of these before I implement it.
BUG: Support assets inside subdirectories