Docs: extends warning

Add a warning to extends keyword to note which keys it doesn't extend

Author: M1ke

docs/pages/product/data-modeling/reference/cube.mdx

@@ -215,6 +215,12 @@ cube(`extended_order_facts`, {
 })
 ```
 
+<WarningBox>
+
+Extends does not apply to the `refresh_key` parameter - cubes which extend another and do not declare a `refresh_key` will use the [default for their given engine][ref-cube-refresh-key].
+
+</WarningBox>
+
 ### `data_source`
 
 Each cube can have its own `data_source` name to support scenarios where data
@@ -636,4 +642,4 @@ The `access_policy` parameter is used to configure [data access policies][ref-re
 [ref-ref-pre-aggs]: /product/data-modeling/reference/pre-aggregations
 [ref-ref-dap]: /product/data-modeling/reference/data-access-policies
 [ref-syntax-cube-sql]: /product/data-modeling/syntax#cubesql-function
-[ref-extension]: /product/data-modeling/concepts/code-reusability-extending-cubes
+[ref-extension]: /product/data-modeling/concepts/code-reusability-extending-cubes