Added/updated documentation for --modules and --modules_dir flags.
Added/updated documentation for --modules and --modules_dir flags.

Review: https://reviews.apache.org/r/47906/

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