passthrough scheme not documented

[rittneje]

As far as I can tell, the passthrough:/// scheme does not appear to be documented anywhere. It is needed in order to properly use the bufconn package.

It is mentioned in this comment: https://github.com/grpc/grpc-go/issues/7091#issuecomment-2037663115

It is not mentioned in the language agnostic gRPC docs. https://github.com/grpc/grpc/blob/master/doc/naming.md