Mention InvalidateBestSize() in DoGetBestClientSize() documentation.

It is not obvious that it needs to be called when the best size changes, so
give at least a hint.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72652 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/window.h

@@ -3547,6 +3547,11 @@ protected:
         GetBestSize() returns an arbitrary hardcoded size for the window, so
         you must override it when implementing a custom window class.
 
+        Notice that the best size returned by this function is cached
+        internally, so if anything that results in the best size changing (e.g.
+        change to the control contents) happens, you need to call
+        InvalidateBestSize() to notify wxWidgets about it.
+
         @see @ref overview_windowsizing
 
         @since 2.9.0