[doc] Follow developer notes, add comment about missing default.

src/net.cpp

@@ -1852,7 +1852,7 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
                     case ConnectionType::ADDR_FETCH:
                     case ConnectionType::FEELER:
                         setConnected.insert(pnode->addr.GetGroup(addrman.m_asmap));
-                }
+                } // no default case, so the compiler can warn about missing cases
             }
         }