Added "About the data" for modules on high volume tx

Added "About the data" for kpi modules.
"Total Cost" and "Cost per transaction" we also add the notes on costs if they are present.

Added "About the data" for transactions per quarter.
Added "About the data" to all tx dashboards.
Merge pull request #461 from alphagov/enhancement/tx-about-info

Added "About the data" for modules on high volume tx