Nit: better documentation of experimental status on x/ packages?

[bufdev]

This can be closed if we decide against it, but in the spirit of tracking issues here, I wanted to get this up re #863

Our existing x/ packages do not have much documentation regarding their experimental status, structure, and how they should be used. On encoding/x and transport/x, we do have some documentation on the experimental status, but not much more. transport/x/cherami and x/yarpcmeta have no documentation, encoding/x/protobuf has a basic godoc string, and transport/x/grpc has a little more than a basic godoc string, while x/config has more documenation, and transport/x/redis has the most documentation, and a message about the experimental status at the bottom of the godoc. What should we do?