docs: Add CategorySurface text.

include/SDL3/SDL_surface.h

@@ -22,7 +22,17 @@
 /**
  * # CategorySurface
  *
- * SDL_Surface definition and management functions.
+ * SDL surfaces are buffers of pixels in system RAM. These are useful for
+ * passing around and manipulating images that are not stored in GPU memory.
+ *
+ * SDL_Surface makes serious efforts to manage images in various formats,
+ * and provides a reasonable toolbox for transforming the data, including
+ * copying between surfaces, filling rectangles in the image data, etc.
+ *
+ * There is also a simple .bmp loader, SDL_LoadBMP(). SDL itself does not
+ * provide loaders for various other file formats, but there are several
+ * excellent external libraries that do, including it's own satellite library
+ * [SDL_image](https://github.com/libsdl-org/SDL_image).
  */
 
 #ifndef SDL_surface_h_