Add help text row to overview.js
Add help text row to overview.js

So that people have a clue how to use it.

Also, allow omission of this row with via 'no_help' argument.

ministryofjustice/metrics-formula,ministryofjustice/metrics-formula,ministryofjustice/metrics-formula,ministryofjustice/metrics-formula,ministryofjustice/metrics-formula