vfio-ccw: update documentation

The vfio-ccw documentation comes from the cover letter of the
original patch submission, which shows in some parts. Give it some
love; in particular:

- Remove/rework statements that make sense in a cover letter, but not
  in regular documentation.
- Fix some typos.
- Describe the current limitations in more detail.

Acked-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>