[Checkers] Nullness checker

[Michael Ernst]

Adam-

> Why can't I write @Nullable R, but instead there's R extends  
> @Nullable Object? Is it because of current implementation, or some  

The distinction between those two syntaxes is explained in section 2.5.1 of
the Checker Framework manual:

http://groups.csail.mit.edu/pag/jsr308/current/checkers-manual.html#polymorphism

Can you take a look and suggest improvements to that section if it is not
clear?

                     Thanks,

                    -Mike