sys/windows: update docs about secure way to load system DLLs

[disconnect3d]

Before this change, the documentation of the syscall.LazyDLL states to use windows.LazyDLL for secure loading of system DLLs. However, the use of the windows.LazyDLL structure is only secure (not vulnerable to DLL preloading/injection attacks) if the user creates the object through the windows.NewLazySystemDLL function or if they use windows.NewLazyDLL with an absolute path.

Given all this, I think it is better to point to windows.NewLazySystemDLL function instead of the windows.LazyDLL structure in the syscall.LazyDLL documentation.

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