spec: clarify that {ext} is mandatory and its possible values for the various URLs types.
spec: clarify that {ext} is mandatory and its possible values for the various URLs types.

As {ext} is needed to discriminate between image URLs and signature URLs
clarify that they are mandatory and its values should be either `aci` (for the
image) or `sig` (for the signature).

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