Improve documentation for stream argument to __dlpack__

[rgommers]

• Closes stream in __dlpack__ should not be required to be integers #183 (SYCL needs a queue not an integer).
• The need to document the ownership of stream came up in Add torch.cuda.streams.ExternalStream pytorch/pytorch#57781.

This is getting pretty long for reference documentation of a method, we may need to split it out to a separate page soon. Especially given that there may be more to add soon:

• a way to communicate API/ABI version information
• how to correctly add a dependency between streams (PushStreamDep in Specify synchronization semantics dmlc/dlpack#57 (comment))
• lifetime of the DLTensor

For now this is the minimal update for things that are being discussed.

Cc @oleksandr-pavlyk, @tqchen, @leofang

[BvB93]

Suggested change

	- **stream**: _Optional\[ int ]_
	- **stream**: _Any_

You could also go for Optional[Union[int, Any]] or Union[int, None, Any], but in the end all these statements are equivalent.

[rgommers]

Thanks Bas, I missed updating that. From a typing point of view they're indeed all the same. From a "this serves as documentation as well" point of view, I think I prefer Optional[Union[int, Any]].

[leofang]

This is getting pretty long for reference documentation of a method, we may need to split it out to a separate page soon.

I agree. By moving the new additions to a separate page (say the current "Data interchange mechanisms"), we can just add a new :::{admonition} block for SYCL to keep the docstring reasonably long.

[rgommers]

By moving the new additions to a separate page (say the current "Data interchange mechanisms")

Makes sense. Let me do that in a separate PR, and address some of the other points in the PR description as well once we've figured out how to communicate version information.

[rgommers]

I propose to merge this as is; the bigger update and content moving will take a little while.

[rgommers]

Thanks for the reviews everyone!