Allow the watermark to always land on the relay parent (#1689)

roadmap/implementers-guide/src/runtime/router.md

@@ -185,7 +185,9 @@ Candidate Acceptance Function:
 * `check_hrmp_watermark(P: ParaId, new_hrmp_watermark)`:
   1. `new_hrmp_watermark` should be strictly greater than the value of `HrmpWatermarks` for `P` (if any).
   1. `new_hrmp_watermark` must not be greater than the context's block number.
-  1. in `HrmpChannelDigests` for `P` an entry with the block number equal to `new_hrmp_watermark` should exist.
+  1. `new_hrmp_watermark` should be either
+      1. equal to the context's block number
+      1. or in `HrmpChannelDigests` for `P` an entry with the block number should exist
 * `verify_outbound_hrmp(sender: ParaId, Vec<OutboundHrmpMessage>)`:
   1. For each horizontal message `M` with the channel `C` identified by `(sender, M.recipient)` check:
       1. exists