Update warehouse-connectors.mdx

Adding context for the 1MB limit on json columns from BQ to GCS step

pages/docs/tracking-methods/warehouse-connectors.mdx

@@ -67,6 +67,7 @@ Navigate to [Project Settings → Warehouse Sources](https://mixpanel.com/report
             TO "<mixpanel-service-account>"
           ```
 
+JSON columns mapped in BigQuery containing multiple properties are subject to a hard limit of 1MB per record. This limitation is imposed by Google Cloud's handling of JSON objects and applies during the intermediate step where data is written to the GCS bucket.
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