doc: Add and fix comments about never destroyed objects

src/sync.cpp

@@ -95,6 +95,8 @@ struct LockData {
 LockData& GetLockData() {
     // This approach guarantees that the object is not destroyed until after its last use.
     // The operating system automatically reclaims all the memory in a program's heap when that program exits.
+    // Since the ~LockData() destructor is never called, the LockData class and all
+    // its subclasses must have implicitly-defined destructors.
     static LockData& lock_data = *new LockData();
     return lock_data;
 }