docs: clarify listSubscriptionsForAContact returns only explicit preferences (#462)

Add a note to the endpoint description clarifying that this API only
returns subscriptions where the contact has explicitly opted-in or
opted-out. Subscriptions in the default state (no explicit choice made)
are not included in the response.

Co-authored-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: marcmlc

descriptions/0/api.intercom.io.yaml

@@ -5123,6 +5123,8 @@ paths:
 
           1.Opt-out subscription types that the user has opted-out from.
           2.Opt-in subscription types that the user has opted-in to receiving.
+
+        **Note:** This endpoint only returns subscriptions where the contact has explicitly configured their preference. Subscriptions that are in the default state — where the contact has not made an explicit opt-in or opt-out choice — are not included in the response.
       responses:
         '200':
           description: Successful