Document in com.sun.javadoc.Doclet class

The standard doclet does not have any special code to support the -source
option. The validation for that option is handled in the javadoc tool. Does
the stubmaker doclet meet these 2 requirements?

1. Ignore unknown options during validation (Doclet.validOptions()).

2. Return a negative number for unknown options when checking "option length"
(Doclet.optionLength()).

I suspect that stubmaker does not meet requirement #1. I can't think of any
other reason why it would not support -source option. We need to improve the
API documentation so that other doclet writers know about this.