define "transit-oriented" header

[dkg]

in #14, @juga0 points out that "transit-oriented" header isn't well-defined. should we add such a definition?

fwiw, i have only a fuzzy definition of this idea myself, and was originally using the idea in only a vague/intuitive sense, hoping that it was only used in an advisory sense, not in an explicit or normative sense. That is, it would be really bad if an undefined term was used to direct implementers to make specific choices. But if it's used mainly for flavor/intuition/insight, maybe it's ok to leave it undefined?