Explain "traits" and their use-cases

[clemensv]

The spec jumps into defining traits without clarifying use-cases for them. I can imagine them being a way to enforce some uniformity across metadata for several messages at once, but the spec doesn't say that.

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[Fannon]

@clemensv We currently have an open PR where we discuss and want to more clearly define the trait behavior. Your feedback on this would be very welcome.

See #532

[fmvilas]

Closing as it's a duplicate of #532.