Add warnings to `approveAndCall(...)` and `receiveApproval(...)`

[[e-mail] lovenotwar.eth]

@pertsev had some comments on approveAndCall(...) and receiveApproval(...) on the error handling and malicious contracts.

I will be keeping the design for approveAndCall(...) and receiveApproval(...) the same, is it is what I intended, but will be adding comments to add some warnings.