Add note about correct _in operator array syntax

docs/graphql-api/queries/filters/comparison-operators.mdx

@@ -472,6 +472,20 @@ Fetch a list of articles rated 1, 3 or 5:
 }`}
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+:::caution Using _in operator with multiple values
+When using the `_in` operator with multiple values in GraphiQL or your application code, always use proper array syntax with square brackets:
+```
+where: { rating: { _in: [1, 3, 5] } }   // CORRECT
+```
+
+Do not use comma-separated strings, as this will not work correctly:
+```
+where: { rating: { _in: "1, 3, 5" } }   // INCORRECT
+```
+
+While GraphiQL may sometimes auto-generate queries using the incorrect string format, these queries will not function properly when executed against the GraphQL server.
+:::
+
 ## Filter or check for null values (\_is_null)
 
 Checking for null values can be achieved using the `_is_null` operator.