docs: clarify helm chart version upgrades
docs: clarify helm chart version upgrades

Since the cluster helm chart was created in v1.7, the new helm
chart specifies the ceph image, so we clarify the upgrade
doc with the latest info.

Signed-off-by: Travis Nielsen <a8b474fdb3f77f44446f0ab9ddca8147b09b3a9e@redhat.com>

rook/rook,rook/rook,rook/rook,rook/rook,rook/rook