docs: Document operator.unmanagedPodWatcher
docs: Document operator.unmanagedPodWatcher

Describe what the new operator.unmanagedPodWatcher option does in the
upgrade guide.

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

michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium