fixup! Revoke fully unmapped reservations

Better document the quarantine path in vm_map_remove_locked.

sys/vm/vm_map.c

@@ -4720,7 +4720,15 @@ vm_map_remove_locked(vm_map_t map, vm_offset_t start, vm_offset_t end)
 #ifdef CHERI_CAPREVOKE
  if (quarantine_unmapped_reservations &&
  start < VM_MAXUSER_ADDRESS) {
+ /*
+ * If we're here, [start, end) was the last mapped
+ * range in reservation and it has transitioned to
+ * fully unmapped with a new entry. Look up the
+ * entry and quarantine it.
+ */
  vm_map_lookup_entry(map, start, &entry);
+ KASSERT(entry->start <= start && entry->end >= end,
+ ("entry doesn't cover removed range"));
  vm_map_entry_quarantine(map, entry);
  } else
 #endif