Document why RAID shouldn't be used with Swift

Based on an email from Chuck Thier on the OpenStack mailing list

https://lists.launchpad.net/openstack/msg19633.html

Patchset subs comma for period (typo fix)

Rebase against master

Cherry picked from https://review.openstack.org/#/c/19256/

Change-Id: I3061d1bb61049da7eae1eb472872c995fd6d7131
Merge "Document why RAID shouldn't be used with Swift" into stable/folsom