doc: clarify that async_hook callbacks cannot be async

Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: #37384
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>

Author: jasnell

doc/api/async_hooks.md

@@ -129,6 +129,10 @@ class MyAddedCallbacks extends MyAsyncCallbacks {
 const asyncHook = async_hooks.createHook(new MyAddedCallbacks());
 ```
 
+Because promises are asynchronous resources whose lifecycle is tracked
+via the async hooks mechanism, the `init()`, `before()`, `after()`, and
+`destroy()` callbacks *must not* be async functions that return promises.
+
 ##### Error handling
 
 If any `AsyncHook` callbacks throw, the application will print the stack trace