We should improve javadoc document of javac, for the benefit of compiler developers.

The compiler has public API, that for the most part is already documented well to the Sun standards for public API. For example, see http://java.sun.com/javase/6/docs/technotes/guides/javac/index.html
(The one issue that comes to mind is the legacy public entry points in com.sun.tools.javac.Main. Arguably we should document that class too.)

However, all those classes are not the issue here. Instead it is proposed to improve the javadoc on the compiler's implementation classes, for the benefit of developers working on the compiler. The generated javadoc would not be part of any public release, but would be readable in the compiler source code and buildable from the langtools build.xml file.

This should include:
update build.xml to generate javadoc for the compiler internal API
overview.html (com/sun/tools/javac?)
package.html for most if not all the packages in com.sun.tools.javac
reasonable class documentation for what might be considered the "primary" top level classes -- main.Main, JavaCompiler, Scanner, Parser, Enter, Attr, etc. To a first approximation the list should include classes that call or are directly called by JavaCompiler.
In time, there should at least be minimal class documentation for all public and protected classes within the compiler.