doc: add stability index to the --watch-kill-signal flag

PR-URL: #58997
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: dario-piotrowicz

doc/api/cli.md

@@ -3108,6 +3108,8 @@ added:
   - v22.18.0
 -->
 
+> Stability: 1.1 - Active Development
+
 Customizes the signal sent to the process on watch mode restarts.
 
 ```bash