Add warning about empty topics in Custom Webhook filters

fern/api-reference/data/webhooks/webhook-types/custom-webhook.mdx

@@ -2409,6 +2409,15 @@ Below you can find descriptions for select response fields. Note that this is no
 
 If you're new to GraphQL, this editor has autocomplete enabled and will magically suggest fields as you start typing. If you get lost, you can always search for available queries in the docs tab on the right!
 
+<Warning>
+**Important Note About Empty Topics**
+
+Leaving topics empty makes your filter match **all events** (for the selected address, or for **all addresses** if none is provided).  
+This can generate very high webhook volume and **significant usage costs**.  
+If you only want specific event types, be sure to include topic filters.  
+Learn more: [Event Log Filters](https://www.alchemy.com/docs/reference/custom-webhook-filters#eventlog-filters)
+</Warning>
+
 6. Test and validate your GraphQL query by clicking the **Test Webhook** button. Confirm that your GraphQL query is valid does not contain any syntactical errors. **NOTE:** If you want to test your GraphQL query on a historical block, you can simply define your target block hash within the top-level block filter.
 
 <CodeGroup>