I have two packages, Shapes and Fruits:
com.myproject.Shapes.
Circle
Square
Triangle
com.myproject.Fruits.
Apple
For another package use this syntax:
{@link package.class#member label}
In your case this should be:
{@link com.myproject.Shapes.Square Square}
If you want to show only the class name then use the label, if complete path is desired then label is not required.
Reference: http://docs.oracle.com/javase/7/docs/technotes/tools/windows/javadoc.html#link