[UPDATED] JetStream go doc in the interface
[UPDATED] JetStream go doc in the interface

Copied the go doc from implementation to interface, and reworded
some sections.

I decided to copy versus move because some IDE (for instance VS code)
seem to show the doc (when hovering) from the implementation, not
the interface.

Signed-off-by: Ivan Kozlovic <a15f8b81a160b4eebe5c84e9e3b65c87b9b2f18e@synadia.com>

nats-io/nats,nats-io/nats