document how java-options values with spaces are processed

when the jpackage option "--java-options" is used with a value containing spaces, the cfg file is ill formed, resulting in an app that may not be able to run.
for example:
jpackage \
--input ../input-jars \
--output output \
--name simple \
--main-jar hello.jar \
--main-class hello \
--java-options '-Dfoo=foo bar' \
results in a simple.cfg file with:
[JavaOptions]
-Dfoo=foo
bar

the app then fails since 'bar' is not a valid option