[FrameworkBundle] improve server:run feedback

In #12253, improved feedback was added to the `server:run` command
instructing the user how to terminate the built-in web server. This
is hereby backported to the `2.3` branch.
minor #12257 [FrameworkBundle] improve server:run feedback (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[FrameworkBundle] improve server:run feedback

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

In #12253, improved feedback was added to the `server:run` command
instructing the user how to terminate the built-in web server. This
is hereby backported to the `2.3` branch.

Commits
-------

264c37a [FrameworkBundle] improve server:run feedback