Add note to aws_shield_proactive_engagement regarding phone number requirement (#42279)

* Add note to regarding phone number requirement

* Update phone number note

Author: justinretzolk

website/docs/r/shield_proactive_engagement.html.markdown

@@ -76,6 +76,8 @@ The following arguments are required:
 
 ### emergency_contacts
 
+~> **Note:** The contacts that you provide here replace any contacts that were already configured within AWS. While `phone_number` is marked as optional, to enable proactive engagement, the contact list must include at least one phone number. If a phone number is not already configured within AWS, one must be provided in order to prevent errors.
+
 * `contact_notes` - (Optional) Additional notes regarding the contact.
 * `email_address` - (Required) A valid email address that will be used for this contact.
 * `phone_number` - (Optional) A phone number, starting with `+` and up to 15 digits that will be used for this contact.