In part 1, I wrapped a JAR into an existing module. Then I associated javadoc and sources with that wrapped JAR. After that, javadoc was shown and browsing into sources worked.
However, wrapping a JAR into an existing module is suboptimal. (That's a clumsy word for 'undersirable'.) It's better to create a completely new (library wrapper) module, to wrap the external JAR. That gives you a new module that you can share with multiple modules. That also gives you a more modular application.
However, now that your external JAR is included in a more correct (i.e., more modular) way, how can you incorporare javadoc and sources to simplify your coding in the IDE?
Answer: Manually pasting this into project.properties had the same result as before (where it was achieved via a dialog made available via the Edit button on the wrapped JAR):
I.e., the difference between having javadoc and 'browse to sources' available and NOT having it available... was having the above two entries in project.properties of the 'xmlworker' library wrapper module.