Gradle's javadoc task should provide more propreties that allow for finer-grained tuning of Javadoc

The current level of tuning offered by Gradle's javadoc task is inadequate.
While it would be nice if every feature were supported, most users expect
the most commonly used features of Javadoc to be available.

Some properties are inherently problematic, like "private" (which is a Groovy keyword),
and some require more back & forth between the underlying Ant.javadoc object than
is convenient w/o the use of properties devoted to passing closures (which might be cool
but might also represent a bit of work to get the interface just right). Still there's a lot of
"low hanging fruit" – commonly needed stuff that's easy to implement.