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
(cherry picked from commit 76a040cb59a921142f2af30ef88ad1180fcaf2f8)
Merge "Comment related to device name in attach volumes" into stable/havana