Updated comments in authorizer.proto.
Updated comments in authorizer.proto.

Review: https://reviews.apache.org/r/48093/

dforsyth/mesos,gsantovena/mesos,reneploetz/mesos,chhsia0/mesos,abudnik/mesos,craimbert/mesos,yuquanshan/customizedMesos,andschwa/mesos,reneploetz/mesos,kaysoky/mesos,abudnik/mesos,neilconway/mesos,neilconway/mesos,craimbert/mesos,Aman-Jain-14/customizedMesos,reneploetz/mesos,kaysoky/mesos,craimbert/mesos,reneploetz/mesos,Gilbert88/mesos,craimbert/mesos,neilconway/mesos,zmalik/mesos,yuquanshan/customizedMesos,verizonlabs/mesos,reneploetz/mesos,chhsia0/mesos,abudnik/mesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,verizonlabs/mesos,yuquanshan/customizedMesos,kaysoky/mesos,zmalik/mesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,abudnik/mesos,chhsia0/mesos,andschwa/mesos,dforsyth/mesos,neilconway/mesos,gsantovena/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,gsantovena/mesos,reneploetz/mesos,asamerh4/mesos,dforsyth/mesos,verizonlabs/mesos,Gilbert88/mesos,chhsia0/mesos,reneploetz/mesos,andschwa/mesos,Gilbert88/mesos,zmalik/mesos,jpeach/mesos,dforsyth/mesos,zmalik/mesos,kaysoky/mesos,neilconway/mesos,craimbert/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,andschwa/mesos,yuquanshan/customizedMesos,jpeach/mesos,chhsia0/mesos,asamerh4/mesos,andschwa/mesos,asamerh4/mesos,yuquanshan/customizedMesos,chhsia0/mesos,zmalik/mesos,jpeach/mesos,Gilbert88/mesos,craimbert/mesos,andschwa/mesos,jpeach/mesos,zmalik/mesos,Gilbert88/mesos,andschwa/mesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,jpeach/mesos,yuquanshan/customizedMesos,abudnik/mesos,dforsyth/mesos,verizonlabs/mesos,kaysoky/mesos,verizonlabs/mesos,abudnik/mesos,verizonlabs/mesos,zmalik/mesos,gsantovena/mesos,Aman-Jain-14/customizedMesos,neilconway/mesos,asamerh4/mesos,craimbert/mesos,yuquanshan/customizedMesos,asamerh4/mesos,chhsia0/mesos,Gilbert88/mesos,gsantovena/mesos,Gilbert88/mesos,asamerh4/mesos,asamerh4/mesos,neilconway/mesos,kaysoky/mesos,jpeach/mesos,jpeach/mesos,abudnik/mesos,dforsyth/mesos