Add a rationale for allowing both inline filters and passing filters by id.

api/client-server/sync.yaml

@@ -34,7 +34,10 @@ paths:
           description: |-
             The ID of a filter created using the filter API or a filter JSON
             object. The server will detect whether it is an ID or a JSON object
-            by whether the first character is a ``"{"`` open brace.
+            by whether the first character is a ``"{"`` open brace. Passing the
+            JSON inline is best suited to one off requests. Creating a filter
+            using the filter API is recommended for clients that reuse the same
+            filter multiple times, for example in long poll requests.
           x-example: "66696p746572"
         - in: query
           name: since