wxTopLevelWindow::GetClientSize() returns 0x0 when the window is minimized under wxGTK too now; documented this behaviour

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@44444 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/window.tex

@@ -873,6 +873,9 @@ Returns the size of the window `client area' in pixels. The client area is the
 area which may be drawn on by the programmer, excluding title bar, border,
 scrollbars, etc.
 
+Note that if this window is a top-level one and it is currently minimized, the
+return size is empty (both width and height are $0$).
+
 \wxheading{Parameters}
 
 \docparam{width}{Receives the client width in pixels.}