Add onLoad gottcha note to next/script doco

Add a note to the onLoad section of the next/script documenation clarifying that onLoad will not run if `strategy="beforeInteractive"`.

Author: callumgare

docs/api-reference/next/script.md

@@ -46,6 +46,8 @@ The loading strategy of the script.
 
 A method that returns additional JavaScript that should be executed after the script has finished loading.
 
+> **Note: onLoad will not fire if the `strategy` prop is set to `beforeInteractive`.**
+
 The following is an example of how to use the `onLoad` property:
 
 ```jsx