configuration-example.yml: add documentation on batch size [CR]

Clarify in documentation that there is a maximum API batch size of 1000.

configuration-example.yaml

@@ -15,7 +15,8 @@ sync:
   inter_batch_delay: 5
 
   # Batch size for Google API calls. Google supports batching requests together
-  # into one API call. Default: 50.
+  # into one API call. This can be no greater than 1000 but in practice this
+  # should be less to avoid hitting other Google rate limits. Default: 50.
   batch_size: 50
 
 # Configure limits defining maximum scope of changes.