docs: Filled in some category pages.

include/SDL3/SDL_video.h

@@ -22,7 +22,27 @@
 /**
  * # CategoryVideo
  *
- * SDL video functions.
+ * SDL's video subsystem is largely interested in abstracting window
+ * management from the underlying operating system. You can create windows,
+ * manage them in various ways, set them fullscreen, and get events when
+ * interesting things happen with them, such as the mouse or keyboard
+ * interacting with a window.
+ *
+ * The video subsystem is also interested in abstracting away some
+ * platform-specific differences in OpenGL: context creation, swapping buffers,
+ * etc. This may be crucial to your app, but also you are not required to use
+ * OpenGL at all.
+ * In fact, SDL can provide rendering to those windows as well, either with an
+ * easy-to-use [2D API](https://wiki.libsdl.org/SDL3/CategoryRender)
+ * or with a more-powerful [GPU API](https://wiki.libsdl.org/SDL3/CategoryGPU).
+ * Of course, it can simply get out of your way and give you the window
+ * handles you need to use Vulkan, Direct3D, Metal, or whatever else you like
+ * directly, too.
+ *
+ * The video subsystem covers a lot of functionality, out of necessity, so it
+ * is worth perusing the list of functions just to see what's available, but
+ * most apps can get by with simply creating a window and listening for
+ * events, so start with SDL_CreateWindow() and SDL_PollEvent().
  */
 
 #ifndef SDL_video_h_