Updated references to online docs in exceptions

@bmunkholm @jpsim 
Some references to the docs from the exceptions were outdated. Also clarified one of them, about arrays.
That sounds a bit better. Also ```The protocol for an RLMArray property must be defined with RLM_ARRAY_TYPE``` is incorrect. You don't have to use the macro to define the protocol. And if a user even made it this far to begin this then they already sucessfully made the call passing in a protocol, so I'm not sure this sentence is even needed?