docs(sdl2): Documentar VSync en renderer

Clarificar el uso de SDL_RENDERER_PRESENTVSYNC:
- Sincroniza con refresco del monitor (60Hz)
- Elimina screen tearing
- Reduce consumo CPU/GPU

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/backend/sdl2.zig

@@ -48,10 +48,9 @@ pub const Sdl2Backend = struct {
         errdefer c.SDL_DestroyWindow(window);
 
         // Create renderer with hardware acceleration and VSync
-        // VSync syncs with monitor refresh rate (typically 60Hz) which:
+        // VSync syncs with monitor refresh rate (typically 60Hz):
         // - Eliminates screen tearing
-        // - Reduces CPU usage (no busy-wait needed)
-        // - Provides consistent frame timing
+        // - Reduces CPU/GPU usage
         // Falls back to software renderer if GPU not available
         const renderer = c.SDL_CreateRenderer(
             window,