Expand documentation for the traits merge mechanism in v3

[smoya]

Imho, it would be very nice to point to a reference implementation or code snippet how this is to be implemented correctly. As a Dev, I would feel more confident looking at a code example to check my understanding. I like the example, but a simple example cannot cover all the interesting corner cases :)

Originally posted by @Fannon in asyncapi/spec#907 (comment)