[cdac] document TypeValidation and MethodValidation

[lambdageek]

The implementations of RuntimeTypeSystem_1 GetTypeHandle and GetMethodHandle perform validation (implemented in the TypeValidation and MethodValidation classes in the cDAC reader) on their input parameters.

While this is not part of the contract, it is an important part of the implementation and we should capture what the validation is doing.

In particular the SOS !U command sometimes passes IP addresses for the methodDesc argument to the ISOSDacInterface::GetMethodDescData method and relies on validation to return an error code

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