[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
             }
         }
 

src/net.h

@@ -874,7 +874,7 @@ public:
             case ConnectionType::ADDR_FETCH:
             case ConnectionType::FEELER:
                 return false;
-        }
+        } // no default case, so the compiler can warn about missing cases
 
         assert(false);
     }
@@ -919,7 +919,7 @@ public:
             case ConnectionType::BLOCK_RELAY:
             case ConnectionType::ADDR_FETCH:
                 return true;
-        }
+        } // no default case, so the compiler can warn about missing cases
 
         assert(false);
     }