SPEC: discovery clarifications
SPEC: discovery clarifications

- codify/define "discovery URL" + template
- remove misleading `?torrent` tag
- remove ACE references, instead talk about discovery clients
- discuss behaviour in case of multiple `ac-discovery` tags

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