Adds clarification to `nomad volume create` commands around how the `volume`
block in the jobspec overrides this behavior. Adds missing section to `nomad
volume register` and to example volume spec for both commands.
The `capacity` block was removed during implementation in lieu of the
`capacity_max` and `capacity_min` fields, but it wasn't removed from the
example in the documentation.
Add new `access_mode`/`attachment_mode` fields. Make it more clear which set
of fields belong to create vs register. Update the example spec that's
generated by `volume init`.
