QQmlData: add documentation

Change-Id: I2e45c2fdf9acef1e0149f2c22a63c6b9fa3adbc8
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/qml/qml/qqmldata_p.h

@@ -118,9 +118,19 @@ public:
         if (!explicitIndestructibleSet) indestructible = false;
     }
 
+    // If ownMemomry is true, the QQmlData was normally allocated. Otherwise it was allocated
+    // with placement new and QQmlData::destroyed is not allowed to free the memory
     quint32 ownMemory:1;
+    // indestructible is set if and only if the object has CppOwnership
+    // This can be explicitly set with QJSEngine::setObjectOwnership
+    // Top level objects generally have CppOwnership (see QQmlcCmponentprivate::beginCreate),
+    // unless created by special methods like the QML component.createObject() function
     quint32 indestructible:1;
+    // indestructible was explicitly set  with setObjectOwnership
+    // or the object is a top-level object
     quint32 explicitIndestructibleSet:1;
+    // set when one QObject has been wrapped into QObjectWrapper in multiple engines
+    // at the same time - a rather rare case
     quint32 hasTaintedV4Object:1;
     quint32 isQueuedForDeletion:1;
     /*
@@ -128,6 +138,7 @@ public:
      * v8 GC will check this flag, only deletes the objects when rootObjectInCreation is false.
      */
     quint32 rootObjectInCreation:1;
+    // set when at least one of the object's properties is intercepted
     quint32 hasInterceptorMetaObject:1;
     quint32 hasVMEMetaObject:1;
     quint32 parentFrozen:1;