Make GRPCTimeout extensible and add documentation. (#689)
Make GRPCTimeout extensible and add documentation. (#689)

Motivation:

Our error types should be extensible to avoid having to do a SemVer
major change to add a new case, this one was missed when other error
types were updated.

Modifications:

Make GRPCTimeout rely on a private enum internally. Add documentation.

Result:

Functionally, no real change.
grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift