doc: clarify that --require only supports cjs

Fixes: https://github.com/nodejs/node/issues/35103

PR-URL: https://github.com/nodejs/node/pull/36806
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: Harshitha K P <harshitha014@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/cli.md

@@ -1211,6 +1211,9 @@ Preload the specified module at startup.
 Follows `require()`'s module resolution
 rules. `module` may be either a path to a file, or a node module name.
 
+Only CommonJS modules are supported. Attempting to preload a
+ES6 Module using `--require` will fail with an error.
+
 ### `-v`, `--version`
 <!-- YAML
 added: v0.1.3