CCM-13640: Update documentation

Author: chris-elliott-nhsd

specification/documentation/CreateMessageBatch.md

@@ -17,7 +17,7 @@ The per message reference (`messageReference`) needs to be unique within the mes
 
 You may be required to send through specific personalisation fields based upon the routing plan (`routingPlanId`). These will have been setup during your [onboarding](#overview--onboarding) process.
 
-These are validated according to the routing plan when we store your message batch. If there are missing values then the entire batch will be rejected with a `CM_INVALID_VALUE` code. The detail and source fields will provide further information about the error.
+Messages that are missing required personalisation fields, or have personalisation content that is too long for the routing plan that it is used in, will be accepted by the API but will not be sent.
 
 ### Sandbox