Doc: Fix documentation warnings for Qt Designer

Add missing documentation for QDesignerFormWindowInterface,
fix \fn commands and missing namespace specifiers.

Change-Id: I3481781d30760768d4b79d8ea889c1c17b251f67
Reviewed-by: Martin Smith <martin.smith@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/designer/src/lib/extension/extension.cpp

@@ -126,7 +126,7 @@ QT_BEGIN_NAMESPACE
 */
 
 /*!
-   \fn T qt_extension(QAbstractExtensionManager* manager, QObject *object)
+   \fn template <class T> T qt_extension(QAbstractExtensionManager* manager, QObject *object)
 
    \relates QExtensionManager
 
@@ -167,7 +167,7 @@ QT_BEGIN_NAMESPACE
    class definition for \a ExtensionName, in the associated header
    file.
 
-   \sa Q_DECLARE_INTERFACE()
+   \sa {Q_DECLARE_INTERFACE}{Q_DECLARE_INTERFACE()}
 */
 
 QT_END_NAMESPACE

src/designer/src/lib/sdk/abstractformwindow.cpp

@@ -820,6 +820,22 @@ void QDesignerFormWindowInterface::activateResourceFilePaths(const QStringList &
     \sa resourceFiles()
 */
 
+/*!
+    \fn ResourceFileSaveMode QDesignerFormWindowInterface::resourceFileSaveMode() const
+
+    Returns the resource file save mode behavior.
+
+    \sa setResourceFileSaveMode()
+*/
+
+/*!
+    \fn void QDesignerFormWindowInterface::setResourceFileSaveMode(ResourceFileSaveMode behavior)
+
+    Sets the resource file save mode \a behavior.
+
+    \sa resourceFileSaveMode()
+*/
+
 /*!
     \fn void QDesignerFormWindowInterface::widgetManaged(QWidget *widget)
 

src/designer/src/lib/sdk/abstractformwindowmanager.cpp

@@ -433,6 +433,12 @@ QAction *QDesignerFormWindowManagerInterface::actionSimplifyLayout() const
    \sa addFormWindow()
 */
 
+/*!
+  \fn QPixmap QDesignerFormWindowManagerInterface::createPreviewPixmap() const
+
+  Creates a pixmap representing the preview of the currently active form.
+*/
+
 /*!
     Allows you to intervene and control \QD's "undo" action. The
     function returns the original action.

src/designer/src/lib/shared/qdesigner_formwindowmanager.cpp

@@ -36,7 +36,7 @@ QT_BEGIN_NAMESPACE
 using namespace qdesigner_internal;
 
 /*!
-    \class QDesignerFormWindowManager
+    \class qdesigner_internal::QDesignerFormWindowManager
     \inmodule QtDesigner
 
     Extends QDesignerFormWindowManagerInterface with methods to control
@@ -58,7 +58,7 @@ QDesignerFormWindowManager::~QDesignerFormWindowManager()
 }
 
 /*!
-    \fn PreviewManager *QDesignerFormWindowManager::previewManager() const
+    \fn PreviewManager *qdesigner_internal::QDesignerFormWindowManager::previewManager() const
 
     Accesses the previewmanager implementation.