docs: Improve upgrade rollback section
docs: Improve upgrade rollback section

This paragraph seemed to be vaguely referencing some section that might
exist in the future at some point, which doesn't make any sense. We can
only refer users to the documentation that exists below. Do so.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

tklauser/cilium,tklauser/cilium,cilium/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,tgraf/cilium,michi-covalent/cilium