[Documentation] Lifetime of device tensors and allocating device tensors with the env allocator

### Describe the documentation issue

When calling [CreateAndRegisterAllocatorV2](https://onnxruntime.ai/docs/api/c/struct_ort_api.html#a12c97c7471565c34f99dac7e574d12cf) is there any way to use the created allocator without an OrtSession ?

A flow that should work is to register the env allocator, create a Session and then acquire the arena allocator (should be the env allocator at this point) through the API shown here. https://onnxruntime.ai/docs/performance/device-tensor.html#cuda

I am thinking about the lifetime of device tensors when using the session allocator, ideally I would syntactically be able to acquire the environment allocator somehow from the environment. That would make it clear that the allocators lifetime is associated with the environment and not just with the session.

### Page / URL

https://onnxruntime.ai/docs/get-started/with-c.html#features