Update wxRibbonPanel documentation to mention use of sizers.

This documents the changes of r65436.

Closes #12418.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65537 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/ribbon/panel.h

@@ -15,6 +15,15 @@
     
     A panel adds a border and label to a group of controls, and can be
     minimised (either automatically to conserve space, or manually by the user).
+
+    Non ribbon controls can be placed on a panel using wxSizers to manage
+    layout. wxWrapSizer and AddStretchSpacer() are useful for proportional
+    vertical and horizontal positioning. Note that layout is done within the
+    constraints of the panel's client area and this is dictated by
+    wxRibbonArtProvider.
+
+    Mixing ribbon and non-ribbon controls in a RibbonPanel is not supported at
+    present.
     
     @sa wxRibbonPage