video: Update the popup window creation documentation and disallowed flags

Update the popup window creation documentation with additional info, remove the SDL_WINDOW_MOUSE_GRABBED flag check since it isn't a valid window creation flag and will be removed automatically elsewhere, and check for and remove the explicit skip taskbar and borderless flags since they are implicit for popup windows.

include/SDL3/SDL_video.h

@@ -667,20 +667,29 @@ extern DECLSPEC SDL_Window *SDLCALL SDL_CreateWindow(const char *title, int w, i
 /**
  * Create a child popup window of the specified parent window.
  *
- * 'flags' **must** contain exactly one of the following: -
- * 'SDL_WINDOW_TOOLTIP': The popup window is a tooltip and will not pass any
- * input events - 'SDL_WINDOW_POPUP_MENU': The popup window is a popup menu
- *
- * The following flags are not valid for popup windows and will be ignored: -
- * 'SDL_WINDOW_MINIMIZED' - 'SDL_WINDOW_MAXIMIZED' - 'SDL_WINDOW_FULLSCREEN' -
- * `SDL_WINDOW_BORDERLESS` - `SDL_WINDOW_MOUSE_GRABBED`
+ * 'flags' **must** contain exactly one of the following:
+ * - 'SDL_WINDOW_TOOLTIP': The popup window is a tooltip and will not pass any
+ *                         input events.
+ * - 'SDL_WINDOW_POPUP_MENU': The popup window is a popup menu. The topmost
+ *                            popup menu will implicitly gain the keyboard focus.
+ *
+ * The following flags are not relevant to popup window creation and will be ignored:
+ * - 'SDL_WINDOW_MINIMIZED'
+ * - 'SDL_WINDOW_MAXIMIZED'
+ * - 'SDL_WINDOW_FULLSCREEN'
+ * - 'SDL_WINDOW_BORDERLESS'
+ * - 'SDL_WINDOW_SKIP_TASKBAR'
  *
  * The parent parameter **must** be non-null and a valid window. The parent of
  * a popup window can be either a regular, toplevel window, or another popup
  * window.
  *
- * Popup windows cannot be minimized, maximized, made fullscreen, or grab the
- * mouse. Attempts to do so will fail.
+ * Popup windows cannot be minimized, maximized, made fullscreen, raised, flash,
+ * be made a modal window, be the parent of a modal window, or grab the mouse
+ * and/or keyboard. Attempts to do so will fail.
+ *
+ * Popup windows implicitly do not have a border/decorations and do not appear
+ * on the taskbar/dock or in lists of windows such as alt-tab menus.
  *
  * If a parent window is hidden, any child popup windows will be recursively
  * hidden as well. Child popup windows not explicitly hidden will be restored
@@ -696,7 +705,8 @@ extern DECLSPEC SDL_Window *SDLCALL SDL_CreateWindow(const char *title, int w, i
  *                 of the parent window, in screen coordinates
  * \param w the width of the window, in screen coordinates
  * \param h the height of the window, in screen coordinates
- * \param flags 0, or one or more SDL_WindowFlags OR'd together
+ * \param flags SDL_WINDOW_TOOLTIP or SDL_WINDOW_POPUP MENU, and zero or more
+ *              additional SDL_WindowFlags OR'd together.
  * \returns the window that was created or NULL on failure; call
  *          SDL_GetError() for more information.
  *

src/video/SDL_video.c

@@ -1953,7 +1953,7 @@ SDL_Window *SDL_CreatePopupWindow(SDL_Window *parent, int offset_x, int offset_y
     }
 
     /* Remove invalid flags */
-    flags &= ~(SDL_WINDOW_MINIMIZED | SDL_WINDOW_MAXIMIZED | SDL_WINDOW_FULLSCREEN | SDL_WINDOW_MOUSE_GRABBED);
+    flags &= ~(SDL_WINDOW_MINIMIZED | SDL_WINDOW_MAXIMIZED | SDL_WINDOW_FULLSCREEN | SDL_WINDOW_BORDERLESS | SDL_WINDOW_SKIP_TASKBAR);
 
     return SDL_CreateWindowInternal(NULL, offset_x, offset_y, w, h, parent, flags);
 }