loadso: library handles are now SDL_SharedObject* instead of void*.

Improved the SDL_loadso.h documentation a little, too.

Fixes #11009.

src/render/direct3d12/SDL_render_d3d12.c

@@ -179,8 +179,8 @@ typedef struct
 // Private renderer data
 typedef struct
 {
-    void *hDXGIMod;
-    void *hD3D12Mod;
+    SDL_SharedObject *hDXGIMod;
+    SDL_SharedObject *hD3D12Mod;
 #if defined(SDL_PLATFORM_XBOXONE) || defined(SDL_PLATFORM_XBOXSERIES)
     UINT64 frameToken;
 #else