doc: fix and clarify --block-device usage in user docs
doc: fix and clarify --block-device usage in user docs

The docs on create a volume-backed server from the command
line were wrong in a few ways:

* The openstack server create command does not currently allow
  booting from a volume where a source image is provided and
  nova creates the volume from the image and uses that volume
  as the root disk. The nova boot command supports that, so the
  docs are updated to call out the nova boot command since that
  is the appropriate command in this case (even the syntax with
  the openstack server create --block-device was wrong).

* When creating a server from a bootable volume with the OSC CLI,
  either the --volume or --block-device options should be used
  for a single volume, but not both. The docs were using both, so
  the latter is dropped and a note is added which links to the CLI
  documentation for more details on --block-device option usage.

Change-Id: I985b870759d6c21ef9357b04f39099c02354f135
Closes-Bug: #1794827

openstack/nova,klmitch/nova,openstack/nova,mahak/nova,rahulunair/nova,openstack/nova,rahulunair/nova,rahulunair/nova,mahak/nova,klmitch/nova,klmitch/nova,klmitch/nova,mahak/nova