Updates to the 'Create and manage volumes' section

Updated to reflect the following changes:
- Updated to 'Create a volume' procedure to include
  * New volumes sources (snapshots and volumes can now be used as sources).
  * Using image as source displays new 'availibility zone' field.
- New procedure to edit a volume.
- Note about new option to extend volumes.
- Minor updates like 'Images & Snapshots' to 'Volume Snapshots'.
Partial-Bug: #1307860

Change-Id: I00b937e462898c3a5f8086b90e33c568b8770561
Merge "Updates to the 'Create and manage volumes' section"