Documenting the defaults in the setter methods has always seemed more natural to me. You already need to have the Javadoc for the setter methods describe how calling the method will affect the generated object, and what limitations there may be on the parameters of the setter method. Documenting the defaults there as well seems reasonable. Also having the default documented in the setter method sometimes makes me realize that I don't need to call the setter method at all.

If you document the defaults in the static method that creates the builder that can make the Javadoc long.  If there are multiple methods to create a builder, you may need to document some of the defaults multiple times. I also generally don't care about all the defaults; I assume that the person writing the builder has set reasonable defaults.

...but if you feel strongly that the documentation belongs in the method that creates the builder, then put it there. I think it's important to document the defaults so users of the builder do not need to read the code.