docs: scope the runbook's capacity promise to multi-node clusters

The intro said at most one node's worth of capacity is out of service
during the roll, which is only true when there is more than one node.
Say so, and point at the single-node note in the per-node step.

docs/upgrade.md

@@ -1,9 +1,10 @@
 # Rolling upgrade runbook
 
 This runbook upgrades a running Agent Substrate install to a new build
-version, one node at a time. No actor loses state, and at most one
-node's worth of capacity is out of service while the rest of the fleet
-keeps serving. It needs `kubectl`, `kubectl ate`, `go run
+version, one node at a time. No actor loses state. On a cluster with
+more than one node, at most one node's worth of capacity is out of
+service while the rest of the fleet keeps serving; a single-node cluster
+stops fully during step 5. It needs `kubectl`, `kubectl ate`, `go run
 ./cmd/ate-setup`, `jq`, and `grpcurl`. The numbered steps are the same
 on every Kubernetes provider; the provider-specific parts sit in their
 own sections, [before](#on-gke) and [after](#after-the-roll-on-gke)