spec: clarify the three url types and how templates are used for discovery.
spec: clarify the three url types and how templates are used for discovery.

Now the possible url types and how templates are used should be inferred
reading the various discovery processes. This patch tries to clarify them.

krnowak/appc-spec,iaguis/appc-spec,tklauser/spec,tklauser/spec,krnowak/appc-spec,endocode/appc-spec,3ofcoins/appc-spec,appc/spec,squeed/spec,alban/appc-spec,zenlinTechnofreak/spec,alban/appc-spec,dustymabe/spec,craimbert/spec,iaguis/appc-spec,yifan-gu/spec,squeed/spec,stufflebear/spec,jonboulle/spec,marineam/spec,apcera/appc-spec,dgonyeo/spec,jonboulle/spec,apcera/appc-spec,squeed/spec,apcera/appc-spec,dustymabe/spec,stufflebear/spec,endocode/appc-spec,zenlinTechnofreak/spec,appc/spec,ChengTiesheng/spec,marineam/spec,endocode/appc-spec,ChengTiesheng/spec,dgonyeo/spec,yifan-gu/spec,craimbert/spec,3ofcoins/appc-spec,marineam/spec,dgonyeo/spec,dustymabe/spec,stufflebear/spec,appc/spec,zenlinTechnofreak/spec,krnowak/appc-spec,alban/appc-spec,yifan-gu/spec,3ofcoins/appc-spec,tklauser/spec,ChengTiesheng/spec,craimbert/spec,iaguis/appc-spec,jonboulle/spec