Document typed errors returned by interfaces

[japem]

Observed behavior

The interfaces defined by the jetstream package (and possibly others though I haven't looked into them) do not document the typed errors that they may return. This makes it difficult to use the error types defined in this package to drive behavior after calling the interface methods.

For example, looking at the code I can see that the main implementation of KeyValue.Create() will return ErrKeyExists if the key exists at the time create is called. This should be documented as a requirement of the interface so consumers can rely on this error type being returned and handle it appropriately.

Expected behavior

Typed errors returned by any method should be documented.

Server and client version

github.com/nats-io/nats.go v1.31.0

Host environment

N/A

Steps to reproduce

N/A