Remove time-relative language from A2A experimental notice

Replace "currently experimental and needs further work" with
"experimental" and "not yet integrated" with "not integrated"
per STYLE.md conventions.

Fixes #24568

Author: lacatoire

content/manuals/ai/docker-agent/reference/cli.md

@@ -57,9 +57,9 @@ $ docker agent serve a2a agent-file|registry-ref
 ```
 
 > [!NOTE]
-> A2A support is currently experimental and needs further work. Tool calls are
+> A2A support is experimental. Tool calls are
 > handled internally and not exposed as separate ADK events. Some ADK features
-> are not yet integrated.
+> are not integrated.
 
 Arguments: