docs: Wrote CategoryVulkan.

include/SDL3/SDL_vulkan.h

@@ -23,6 +23,19 @@
  * # CategoryVulkan
  *
  * Functions for creating Vulkan surfaces on SDL windows.
+ *
+ * For the most part, Vulkan operates independent of SDL, but it benefits from
+ * a little support during setup.
+ *
+ * Use SDL_Vulkan_GetInstanceExtensions() to get platform-specific bits for
+ * creating a VkInstance, then SDL_Vulkan_GetVkGetInstanceProcAddr() to get
+ * the appropriate function for querying Vulkan entry points. Then
+ * SDL_Vulkan_CreateSurface() will get you the final pieces you need to
+ * prepare for rendering into an SDL_Window with Vulkan.
+ *
+ * Unlike OpenGL, most of the details of "context" creation and window buffer
+ * swapping are handled by the Vulkan API directly, so SDL doesn't provide
+ * Vulkan equivalents of SDL_GL_SwapWindow(), etc; they aren't necessary.
  */
 
 #ifndef SDL_vulkan_h_