UseJava.cmake: document add_jar compat shim

Document the logic that parses for backward compatibility the old
variables that were used to control add_jar prior to named argument
support. In particular, document that the reason this logic exists is
for backward compatibility, and that new features do not need to add to
it.
Merge topic 'document-add_jar-compat-shims' into next

b41771d UseJava.cmake: document add_jar compat shim