Docs: improve doc-strings for window size & startup file

Document the purpose of zero sized window & when wm_init_state is used.
Noticed when looking into #108643.

source/blender/makesdna/DNA_windowmanager_types.h

@@ -267,8 +267,19 @@ typedef struct wmWindow {
 
   /** Window-ID also in screens, is for retrieving this window after read. */
   int winid;
-  /** Window coords. */
-  short posx, posy, sizex, sizey;
+  /** Window coords (in pixels). */
+  short posx, posy;
+  /**
+   * Window size (in pixels).
+   *
+   * \note Loading a window typically uses the size & position saved in the blend-file,
+   * there is an exception for startup files which works as follows:
+   * Setting the window size to zero before `ghostwin` has been set has a special meaning,
+   * it causes the window size to be initialized to `wm_init_state.size_x` (& `size_y`).
+   * These default to the main screen size but can be overridden by the `--window-geometry`
+   * command line argument.
+   */
+  short sizex, sizey;
   /** Normal, maximized, full-screen, #GHOST_TWindowState. */
   char windowstate;
   /** Set to 1 if an active window, for quick rejects. */

source/blender/windowmanager/intern/wm_window.c

@@ -105,7 +105,16 @@ typedef enum eWinOverrideFlag {
  * These values are typically set by command line arguments.
  */
 static struct WMInitStruct {
-  /* window geometry */
+  /**
+   * Window geometry:
+   * - Defaults to the main screen-size.
+   * - May be set by the `--window-geometry` argument,
+   *   which also forces these values to be used by setting #WIN_OVERRIDE_GEOM.
+   * - When #wmWindow::size_x is zero, these values are used as a fallback,
+   *   needed so the #BLENDER_STARTUP_FILE loads at the size of the users main-screen
+   *   instead of the size stored in the factory startup.
+   *   Otherwise the window geometry saved in the blend-file is used and these values are ignored.
+   */
   int size_x, size_y;
   int start_x, start_y;