gmem.c: Update g_clear_pointer() documentation

Hi,

This attempts to update the documentation for g_clear_pointer() to give examples where calling convention mismatches will cause crashes, such as using Windows Win32 API and OpenGL and Vulkan calls on 32-bit Windows, which typically use __stdcall rather than the normal __cdecl calling convention.

I think recent GTK-4.x crashes on 32-bit Windows caused by this kind of issue merit adding some elaboration to the calling convention part, such as gtk!7294 (merged).

With blessings, thank you!