Disk config manual as 277

Added two notes about diskConfig=MANUAL being required for non-RAX servers. Fixed some typos. This fixes AS-277
Ok, apologies, I actually checked with Manish and it's not just all customer custom images (the snapshotting creates an image which shouldn't trigger this image).

We think maybe this is a problem with imported images, using http://docs.rackspace.com/images/api/v2/ci-gettingstarted/content/import-image.html?  So instead of "you created by snapshotting a non-Rackspace server." would it make more sense to change this text to say "imported using <that above link>" instead?