Clarify cookie link guidance

It has been pointed out that 767062fd altered the intention of the
guidance.

This change reverts that alteration, whilst retaining the style changes.
Perhaps, @alextea, although the service manual isn't prescriptive as a rule-of-thumb. Adding that in there makes it seem that we're suggesting that every Gov't service should use `govuk_template` to do this. As it stands, the manual makes them aware of the requirement but doesn't specify how exactly they implement it.