doc: clarify that --require only supports cjs

Fixes: #35103

PR-URL: #36806
Reviewed-By: Myles Borins <[email protected]>
Reviewed-By: Antoine du Hamel <[email protected]>
Reviewed-By: Beth Griggs <[email protected]>
Reviewed-By: Harshitha K P <[email protected]>
Reviewed-By: Daijiro Wachi <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>

Author: jasnell

doc/api/cli.md

@@ -1128,6 +1128,9 @@ 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