docs: require monotone ceilings in UsageLimitPolicy contract

The dispatch loop stops at the first gated band, so a lower band whose
ceiling exceeds a higher band's can be marked open on calls where it is
never reached and starve. State this requirement in the interface
contract so implementations do not have to discover it from the
dispatch loop.

Signed-off-by: Luke Van Drie <lukevandrie@google.com>

Author: LukeAVanDrie

pkg/epp/framework/interface/flowcontrol/plugins.go

@@ -143,9 +143,14 @@ type SaturationDetector interface {
 // Integration:
 // This policy is called during dispatch decision-making, before a request is allowed to proceed. For each
 // priority band, the returned ceiling is compared against current saturation. If saturation exceeds the
-// ceiling for a given priority, requests at that priority are gated (not dispatched).
-//
-// Conformance: Implementations MUST ensure all methods are goroutine-safe.
+// ceiling for a given priority, requests at that priority are gated (not dispatched). The dispatch loop
+// visits bands from highest to lowest priority and stops at the first gated band; lower bands are not
+// considered on that call.
+//
+// Conformance: Implementations MUST ensure all methods are goroutine-safe. Returned ceilings MUST be
+// monotonically non-increasing in the given priority order: because the dispatch loop stops at the first
+// gated band, a lower band whose ceiling exceeds that of a higher band can be marked open on calls where
+// it is unreachable, starving it.
 type UsageLimitPolicy interface {
 	plugin.Plugin