Fix documentation of "must be vacuumed within" warning.
Warnings start 10M transactions before xidStopLimit, which is 11M
transactions before wraparound. The sample WARNING output showed a
value greater than 11M, and its HINT message predated commit
25ec228ef760eb91c094cc3b6dea7257cc22ffb5. Hence, the sample was
impossible. Back-patch to 9.5 (all supported versions).