doc: Add and fix comments about never destroyed objects

Author: hebasto

src/logging.cpp

@@ -22,8 +22,8 @@ BCLog::Logger& LogInstance()
  * access the logger. When the shutdown sequence is fully audited and tested,
  * explicit destruction of these objects can be implemented by changing this
  * from a raw pointer to a std::unique_ptr.
- * Since the destructor is never called, the logger and all its members must
- * have a trivial destructor.
+ * Since the ~Logger() destructor is never called, the Logger class and all
+ * its subclasses must have implicitly-defined destructors.
  *
  * This method of initialization was originally introduced in
  * ee3374234c60aba2cc4c5cd5cac1c0aefc2d817c.

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;
 }