Document which methods emit code in JastAdd2

[Jesper Öqvist]

It can be difficult to distinguish code generating methods from pure AST and attribute analysis methods in the JastAdd2 source, so we should try to document this and probably separate them into different aspects files.

For example a method like ASTNode.emitImplicitASTNode is a code generating method, while AttrDecl.isPrimitive analyses an attribute and does not generate code.