doc: caveat unavailability of import.meta.resolve in custom

PR-URL: nodejs/node#49242
Backport-PR-URL: nodejs/node#50669
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>

graal-nodejs/doc/api/esm.md

@@ -375,6 +375,9 @@ behind the `--experimental-import-meta-resolve` flag:
 
 * `parent` {string|URL} An optional absolute parent module URL to resolve from.
 
+> **Caveat** This feature is not available within custom loaders (it would
+> create a deadlock).
+
 ## Interoperability with CommonJS
 
 ### `import` statements