Create documentation section for scopes as part of a capability expression

[paddybyers]

Internal: https://ably-real-time.slack.com/archives/CF7GGBF2N/p1582450929004400

Increasingly, as API streamer is exposed more widely, we are finding that it is difficult for end-users to understand the meaning of a capability expression and, in particular, the meaning of scopes within those expressions, and wildcards (whether part of the scope, or as the resource part of a scoped or unscoped capability).

We need some clear documentation to address this.

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