Evas filters: Add a note to the transform doc

This is a special operation, with a simplistic implementation.

src/lib/evas/filters/evas_filter_parser.c

@@ -1565,6 +1565,7 @@ _transform_padding_update(Evas_Filter_Program *pgm,
   Apply a geometrical transformation to a buffer.
 
   Right now, only <b>vertical flip</b> is implemented and available.
+  This operation does not blend and assumes the destination buffer is empty.
 
   @code
     transform (dst, op = vflip, src = input, oy = 0);

src/lib/evas/filters/evas_filter_transform.c

@@ -1,5 +1,12 @@
 #include "evas_filter_private.h"
 
+/* Apply geomeetrical transformations to a buffer.
+ * This filter is a bit simplistic at the moment.
+ *
+ * It also assumes the destination is empty, as it does not use blend
+ * operations. This should probably be fixed later on (use evas_map?).
+ */
+
 static Eina_Bool
 _vflip_cpu(Evas_Filter_Command *cmd)
 {