Inline plugin descriptor referring application classes

This is with reference to the issue discussed here: http://markmail.org/message/z56azjqhfb6bswlz

Greame confirmed that it was buggy of Grails 1.3.7 that it allowed descriptors of plugins to refer to application classes. This behavior is different in Grails 2.0.0.M2 that (correctly) prohibits it.

However, to avoid more users not knowing whether it is a regression or bug-fix compared to Grails <= 1.3.7, the documentation needs to make a note of it at a relevant place.

I am attaching the sample project here that highlighted the difference in behavior.