Document that SetStatusText() updates the display immediately

This is done by explicitly calling Update() in the generic version and
seems to also be done by the native MSW one, even if it's not documented
to do it.

interface/wx/frame.h

@@ -427,7 +427,11 @@ public:
     void SetStatusBarPane(int n);
 
     /**
-        Sets the status bar text and redraws the status bar.
+        Sets the status bar text and updates the status bar display.
+
+        This is a simple wrapper for wxStatusBar::SetStatusText() which doesn't
+        do anything if the frame has no status bar, i.e. GetStatusBar() returns
+        @NULL.
 
         @param text
             The text for the status field.