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