Document what used_now and used_max mean

The "nova host-describe" documentation in the user-guide
doesn't currently explain what used_now and used_max mean.
This commit adds a short explanation of each one to help
users understand the difference between the two values.

Fixes bug 1236430

Change-Id: Id6ab2ddbd0d21b1cd34881d3881d620dc5e060a7
doc xen support for auto_disk_config=disabled

Previously, only True and False were supported. Havana changed to
allows an administrator to specify which images should reject
any user requests for auto config.

Also fix an incorrect statement that auto_disk_config was a nova.conf
variable - it is instead a property set on an image in glance.

Change-Id: I32a09ce1258893388d31ca2c1704e57d23ceff97
Closes-Bug: 1214816
Merge "Document what used_now and used_max mean"
Merge "doc xen support for auto_disk_config=disabled"