It's not stricly required, but I do write it anyway.
I write Javadoc for pretty much everything (including some class member variables) and have
ANT set to generate for protected and more public only.
That should suffice the requirements (in fact it goes over them) as well as showing best practice (and working for the maintainability of the code, which depends on good documentation and coding style, which IMO includes complete developer documentation of private methods as well).
For the Javadoc style to follow, read the style guides and the JDK Javadoc.
There are conventions to be followed for things like the order in which exceptions are listed in Javadoc comments for example, which I'd not seen before.