doc: mark --env-file-if-exists flag as experimental

As `--env-file` is experimental, this must be marked as experimental as
well.

Refs: nodejs#56887
Signed-off-by: Juan José Arboleda <[email protected]>
PR-URL: nodejs#56893
Reviewed-By: Ulises Gascón <[email protected]>
Reviewed-By: Marco Ippolito <[email protected]>

Author: juanarbol

doc/api/cli.md

@@ -800,6 +800,8 @@ node --entry-url 'data:text/javascript,console.log("Hello")'
 
 ### `--env-file-if-exists=config`
 
+> Stability: 1.1 - Active development
+
 <!-- YAML
 added: v22.9.0
 -->