Fixes#6476 - Show message if JVM args are present but new JVM is spawned
* Improved documentation by correctly redacting out `jetty-halt.xml`,
an XML file that is only necessary for rendering the documentation.
* Added WARN message when new JVM is spawned.
* Updated documentation.
* Updated --list-config to report whether a JVM is forked.
* Added test case.
Signed-off-by: Simone Bordet <simone.bordet@gmail.com>