documented need of read-write image handler

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_2_BRANCH@7159 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/fs_mem.tex

@@ -87,6 +87,8 @@ data (bitmap, text or raw data)
 will be copied into private memory stream and available under 
 name "memory:" + filename.
 
+Note that when storing image/bitmap, you must use image format that wxWindows
+can write (e.g. JPG, PNG, see \helpref{wxImage documentation}{wximage})!
 
 
 \membersection{wxMemoryFSHandler::RemoveFile}\label{wxmemoryfshandlerremovefile}