Updated role documentation.
Updated role documentation.

Added information on the distinction between roles and principals.

Review: https://reviews.apache.org/r/43200/

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