Update documentation regarding central widget (#589)

Author: tmartsum

doc/ads_qt_marketplace_description.md

@@ -12,10 +12,10 @@ styles as much as possible.
 
 ## Features
 
-### Docking everywhere - no central widget
+### Docking everywhere - with or without a central widget
 
-There is no central widget like in the Qt docking system. You can dock on every
-border of the main window or you can dock into each dock area - so you are
+The Advanced Docking System works with or without a central widget.
+You can dock on every border of the main window or you can dock into each dock area - so you are
 free to dock almost everywhere.
 
 ![Dropping widgets](https://raw.githubusercontent.com/githubuser0xFFFF/Qt-Advanced-Docking-System/master/doc/preview-dragndrop.png)

doc/user-guide.md

@@ -303,8 +303,9 @@ or to close it via the close button.
 
 ![HideSingleCentralWidgetTitleBar true](cfg_flag_HideSingleCentralWidgetTitleBar_true.png)
 
-The Advanced Docking System is meant for applications without a static central
-widget and normally does not know anything about a central static widget.
+Unless a central widget explicitly has been set with setCentralWidget, the
+Advanced Docking System is without a static central widget and it wouldn't know
+about a central static widget.
 Therefore this flag is disabled by default and a central single dock widget
 still has a titlebar to drag it out of the main window.