Clarify channel, privileges list for same API Key is possible via token auth only

[Srushtika]

The background to this is a conversation in Slack.

It seems that for the sake of simplicity the website dashboard doesn't allow users to list different channels with different privileges under the same API key. However, doing this is possible via Token auth, https://www.ably.io/documentation/core-features/authentication#capabilities-example-key. This information is not clear from the docs and it seems that users are confused as to how they can add a list of channels and privileges via the dashboard. So, the docs need to explain this in a bit more detail.

┆Issue is synchronized with this Jira Task by Unito