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

PR-URL: #58997
Reviewed-By: Rafael Gonzaga <[email protected]>
Reviewed-By: Moshe Atlow <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>

Author: dario-piotrowicz

doc/api/cli.md

@@ -3120,6 +3120,8 @@ added:
   - v24.4.0
 -->
 
+> Stability: 1.1 - Active Development
+
 Customizes the signal sent to the process on watch mode restarts.
 
 ```bash