docs: Update formatstorage to match code for pool path

docs: Update formatstorage to match code for pool path

Seems commit id '0257d06b' forgot to include formatstorage when updating
the docs to describe allowing zfs as a pool type and to furthermore note
that the pool's target path element will be generated rather than read.

Similarly commit id 'efab27afb' neglected to indicate that the target path
for a logical pool will now be generated by libvirt.

Signed-off-by: John Ferlan <jferlan@redhat.com>