Add doc blurb on Cinder pools for NetApp driver

This change adds a statement noting the change in behavior in the NetApp
driver due to the addition of support for the new Cinder pools feature
introduced in Juno.

It also changes the link for the NetApp community site which has
changed.

Change-Id: Id2875e88137901fe06f43bdf82a313c48be6b4a6
Merge "Add doc blurb on Cinder pools for NetApp driver"