Document the meaning of MVCC READ CONFLICT validation code

Fabric doesn't document any of the validation codes meaning that application developers have to try to find information about their meaning somehow and the actions to take if a code is received. This would be a lot of work to do however the most common failure validation code is MVCC READ CONFLICT and as such at least this should be documented and a pointer to a fabric sample that gives an example of dealing with that specific validation code.