Clarify TZIP-12 to point to TZIP-21
Some people (:cough: me) mistook these keywords as defacto standards instead of realizing that TZIP-21 clearly defined them.
Based on discussion here we are going to link to TZIP-21, rather than provide details.
Note that there's still some discussion over whether we should move larger amounts of TZIP-12 into TZIP-21, and that the link won't work until TZIP-21 is merged (which I am inclined to believe will happen imminently). I'm in favor of making this change now, since it stems the immediate bleeding and makes readers of TZIP-12 aware of the numerous new fields ands standards described in TZIP-21.
Phrasing is thanks to @tezit.
Edited by Keefer Taylor