DMP follow-ups (#1875)

* remove pending TODO after the DMP impl merge

* DMP: Update the impl guide

* DMP: Incorporate XCM related changes into the guide

This is the DMP related part of https://github.com/paritytech/polkadot/issues/1702

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

@@ -46,7 +46,7 @@ DownwardMessageQueues: map ParaId => Vec<InboundDownwardMessage>;
 /// - `prev_head`: is the previous head hash or zero if none.
 /// - `B`: is the relay-chain block number in which a message was appended.
 /// - `H(M)`: is the hash of the message being appended.
-DownwardMessageQueueHeads: map ParaId => Option<Hash>;
+DownwardMessageQueueHeads: map ParaId => Hash;
 ```
 
 ### HRMP