Request expanded documentation on implementing `trait Header`

[coriolinus]

The impl Header for Dnt example is a good start, but it doesn't explain the actual semantics of the iterator-based design.

Example questions which would be nice to have answered in the documentation:

• Under what circumstances will the iterator provided to fn decode contain more than one value?
• If a decode implementation does not consume the provided iterator entirely, what happens to the remaining values?
  • Are they re-used to produce a new instance?
  • Are they just discarded?
• Under what circumstances is it reasonable to emit more than one value in fn encode?
• Are the encode and decode circumstances symmetrical?