cleanup and clarify provider spec
cleanup and clarify provider spec

bsmr-erlang/relx,bitwalker/relx,Licenser/relx,priestjim/relx,ethercrow/relx,erlware/relx,timclassic/relx,umatomba/relx,ingwinlu/relx,syl20bnr/relx,RJ/relx,zugolosian/relx,tsloughter/relx,lrascao/relx