Document behavior of compat when creating qcow2 volumes

Document behavior of compat when creating qcow2 volumes

Commit bab2eda changed the behavior for missing compat attribute,
but failed to update the documentation.

Before, the option was omitted from qemu-img command line and the
qemu-img default was used. Now we always specify the compat value
and the default is 0.10.

Reported by Christophe Fergeau
https://bugzilla.gnome.org/show_bug.cgi?id=746660#c4

(cherry picked from commit 7c8ae42d495d683f79ae4baf024f29f089eaa573)
Signed-off-by: Ján Tomko <jtomko@redhat.com>