[EDITORIAL] propose clarification of overloading visitor

[ericprud]

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p11 has:

this means that you can define only the method to handle a certain type of nodes (e.g. field declarations), while your visitor will not do anything with the dozens of other node types you are not interested in.

propose:

this means you only need to defined the methods for nodes you are interested in (e.g. field declarations); the functions inherited from VoidVistorAdaptor will handle the recursion through the rest of the compliation unit's abstract syntax tree.