Docs: Make clear distinction between unsupported inner and supported nested classes

According to http://groovy.codehaus.org/Differences+from+Java, "Inner classes are not supported at the moment." Not every Java developer is necessarily aware of the distinction [1] between an inner class (aka nested non-static class) and a static nested class so it would be advise to make it clear on the page that static nested classes are fully supported by Groovy (and inner classes can be replaced with closures, as mentioned in the docs - it would be actually good to turn that mention into a link to [2] for the cases where you e.g. need to supply an EventListener instance to a Java method call).

References:
[1] http://download.oracle.com/javase/tutorial/java/javaOO/nested.html
[2] http://docs.codehaus.org/display/GROOVY/Groovy+way+to+implement+interfaces