Cleanup: Add comment to wmWindow about pixelsize

Performing operations based on window size in terms of pixel units has a
high chance of resulting in unwanted behavior for macOS, due to HiDPI
displays.

This commit adds an API warning suggesting against usage of the raw
`sizex` and `sizey` values.

Related to #141982

Pull Request: https://projects.blender.org/blender/blender/pulls/142274

source/blender/makesdna/DNA_windowmanager_types.h

@@ -312,6 +312,10 @@ typedef struct wmWindow {
    * it causes the window size to be initialized to `wm_init_state.size`.
    * These default to the main screen size but can be overridden by the `--window-geometry`
    * command line argument.
+   *
+   * \warning Using these values directly can result in errors on macOS due to HiDPI displays
+   * influencing the window native pixel size. See #WM_window_native_pixel_size for a general use
+   * alternative.
    */
   short sizex, sizey;
   /** Normal, maximized, full-screen, #GHOST_TWindowState. */