doc: Add and edit some comments around assumeutxo

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

src/net_processing.cpp

@@ -1937,6 +1937,8 @@ void PeerManagerImpl::BlockConnected(
         }
     }
 
+    // The following task can be skipped since we don't maintain a mempool for
+    // the ibd/background chainstate.
     if (role == ChainstateRole::BACKGROUND) {
         return;
     }