There is no separate set of Javadoc for the NetBeans Platform. However, as the Platform is just a subset of the IDE, the Javadoc for the IDE will apply.
You can browse the Javadoc online (this link always points to the latest development version).
You can also download the Javadoc for a particular NetBeans release:
Finally, you can go to the update center (Tools > Plugin Manager) in the NetBeans IDE and request the NetBeans API Documentation module, which bundles Javadoc matching your IDE release. Use View > Documentation Indices (Help > Javadoc References in NetBeans 6.x) to see the overview pages for API sets currently used by your open projects. The IDE should also automatically display this Javadoc in code completion popups.
Applies to: NetBeans 5.0, 5.5, 6.0, 6.1
Platforms: all