Document allowed characters for KV keys

[jasonmccallister]

When trying to store a KV item on a bucket, using a : throws an error that it is not a valid key. An example key that throws an error is config:key.

What is a valid key is not documented here:
https://github.com/nats-io/nats.docs/blob/master/nats-concepts/jetstream/key-value-store/readme.md

[ahirner]

FWIW, if clients don't enforce it (like nats-py), storing values with keys that contain e.g. & succeeds. Also, watching them with wildcards succeeds with nats cli.

This ADR mentions allowed chars: https://github.com/nats-io/nats-architecture-and-design/blob/main/adr/ADR-8.md

Is there anything in the storage layer, or a planned storage layer that disallows valid subjects as valid keys?