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 <soyjuanarbol@gmail.com>
PR-URL: nodejs#56893
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>

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
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