[Trivial] add comment about setting peer as HB peer.

This adds a comment to the new logic for setting HB peers based
on block validation (and aligns the code below to reflect the comment).
It's not obvious why we're checking mapBlocksInFlight. Add a comment to
explain.

Author: jnewbery

src/net_processing.cpp

@@ -865,7 +865,15 @@ void PeerLogicValidation::BlockChecked(const CBlock& block, const CValidationSta
                 Misbehaving(it->second.first, nDoS);
         }
     }
-    else if (state.IsValid() && !IsInitialBlockDownload() && mapBlocksInFlight.count(hash) == mapBlocksInFlight.size()) {
+    // Check that:
+    // 1. The block is valid
+    // 2. We're not in initial block download
+    // 3. This is currently the best block we're aware of. We haven't updated
+    //    the tip yet so we have no way to check this directly here. Instead we
+    //    just check that there are currently no other blocks in flight.
+    else if (state.IsValid() &&
+             !IsInitialBlockDownload() &&
+             mapBlocksInFlight.count(hash) == mapBlocksInFlight.size()) {
         if (it != mapBlockSource.end()) {
             MaybeSetPeerAsAnnouncingHeaderAndIDs(it->second.first, *connman);
         }