Docs: show more relevant and correct way of using Q_FLAG

The snippet didn't quote the QLibrary header correctly, and didn't
register the flags type, but only the enum type with the meta object
system.

Update example to use QItemSelectionModel instead as a more relevant
class for readers, and restructure the text a bit.

Change-Id: I572e2aaac4601087e7aa6d2ea7a8f8fd65d82539
Fixes: QTBUG-83474
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>

src/corelib/doc/snippets/code/src_corelib_kernel_qobject.cpp

@@ -403,20 +403,28 @@ public:
 
 
 //! [39]
-class QLibrary : public QObject
+class QItemSelectionModel : public QObject
 {
     Q_OBJECT
 
 public:
     ...
-
-    enum LoadHint {
-        ResolveAllSymbolsHint = 0x01,
-        ExportExternalSymbolsHint = 0x02,
-        LoadArchiveMemberHint = 0x04
+    enum SelectionFlag {
+        NoUpdate       = 0x0000,
+        Clear          = 0x0001,
+        Select         = 0x0002,
+        Deselect       = 0x0004,
+        Toggle         = 0x0008,
+        Current        = 0x0010,
+        Rows           = 0x0020,
+        Columns        = 0x0040,
+        SelectCurrent  = Select | Current,
+        ToggleCurrent  = Toggle | Current,
+        ClearAndSelect = Clear | Select
     };
-    Q_DECLARE_FLAGS(LoadHints, LoadHint)
-    Q_FLAG(LoadHint)
+
+    Q_DECLARE_FLAGS(SelectionFlags, SelectionFlag)
+    Q_FLAG(SelectionFlags)
     ...
 }
 //! [39]

src/corelib/kernel/qobject.cpp

@@ -4493,16 +4493,15 @@ QDebug operator<<(QDebug dbg, const QObject *o)
     that values of a given enum can be used as flags and combined using the
     bitwise OR operator. For namespaces use \l Q_FLAG_NS() instead.
 
-    The macro must be placed after the enum declaration.
+    The macro must be placed after the enum declaration. The declaration of
+    the flags type is done using the \l Q_DECLARE_FLAGS() macro.
 
-    For example, in QLibrary, the \l{QLibrary::LoadHints}{LoadHints} flag is
+    For example, in QItemSelectionModel, the
+    \l{QItemSelectionModel::SelectionFlags}{SelectionFlags} flag is
     declared in the following way:
 
     \snippet code/src_corelib_kernel_qobject.cpp 39
 
-    The declaration of the flags themselves is performed in the public section
-    of the QLibrary class itself, using the \l Q_DECLARE_FLAGS() macro.
-
     \note The Q_FLAG macro takes care of registering individual flag values
     with the meta-object system, so it is unnecessary to use Q_ENUM()
     in addition to this macro.