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

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

Refs: #56887
Signed-off-by: Juan José Arboleda <soyjuanarbol@gmail.com>
PR-URL: #56893
Backport-PR-URL: #56932
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>

Author: juanarbol

doc/api/cli.md

@@ -1689,6 +1689,8 @@ Node.js which is `nodejs_conf` and is default when this option is not used.
 added: REPLACEME
 -->
 
+> Stability: 1.1 - Active development
+
 Behavior is the same as [`--env-file`][], but an error is not thrown if the file
 does not exist.