spec: clarify discovery validation/authentication
spec: clarify discovery validation/authentication

Separately call out signature validation, and add an example.
Other minor tweaks to wording for clarification.

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