docs: require scheme for TBdev gRPC API endpoint (#4669)
docs: require scheme for TBdev gRPC API endpoint (#4669)

Summary:
The [gRPC docs] indicate that the default scheme for a channel address
is `dns:///`, but inside Google this is not always the case (Googlers,
see <http://b/179805849>.) Thus, we now require that the API endpoint to
the TensorBoard.dev gRPC server include an explicit scheme. The current
prod servers don’t follow this, but they will soon following internal
changes.

[gRPC docs]: https://github.com/grpc/grpc/blob/master/doc/naming.md

wchargin-branch: docs-grpc-scheme

tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard