Added high-level description to persistent volume docs.
Added high-level description to persistent volume docs.

Review: https://reviews.apache.org/r/38325

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