Comment related to device name in attach volumes

Comment in "Attach Volumes to instances" ("Manage volumes" section), in
order to inform that device name may not match the one proposed by the user.
This limitation is related to hypervisor settings.
The documentation of this issue was recommended by John Griffith in
https://review.openstack.org/#/c/60257/

backport: havana
Related-Bug: #1217874

Change-Id: Ifa65ec6eecca23e85786b0fd79ae3b8ef8ebd537
Merge "Comment related to device name in attach volumes"