PDS enrichment clarification content:

- added content to /routing-plans that clarifies that we do not re-enrich contact details everytime a fallback occurs

Author: jakecosgrove

docs/pages/using-nhs-notify/routing-plans.md

@@ -20,6 +20,14 @@ You can set up routing plans to:
 - use specific message templates for different groups of recipients
 - stop sending messages to a recipient when a channel is successful
 
+## Routing plans and contact details from the Personal Demographics Service (PDS)
+
+NHS Notify checks for each recipient's contact details before any messages are sent using a routing plan.
+
+This means that if a recipient's contact details are not found for a message channel, that channel is automatically skipped.
+
+We do not recheck for contact details each time there's a fallback in a routing plan.
+
 ## Message cascades
 
 A message cascade allows you to message a recipient with different channels in the order you choose. You can set up primary and secondary message cascades in your routing plan.