[doc][rdt] Add the limitations of rdt

[Qiaolin-Yu]

Description

rdt currently has some limitations. update it in the doc to clarify. Disable some tests for the new assertion.

Related issues

Additional information

[gemini-code-assist]

Code Review

This pull request adds a documentation note about a limitation of Ray Direct Transport (RDT) with NIXL. The change is clear and helps inform users about a known issue. I have one minor suggestion to improve the wording for better clarity and professionalism.

[gemini-code-assist]

The phrase "We will fix this problem soon" is informal and vague. It's better to remove it for conciseness and professionalism, especially since the introduction to this section already states that limitations may be addressed in future releases.

Suggested change

	* Due to an issue with our implementation of memory deregistration, we currently do not support repeated transfers of tensors that share the same memory space but belong to different objects. We will fix this problem soon.
	* Due to an issue with our implementation of memory deregistration, we currently do not support repeated transfers of tensors that share the same memory space but belong to different objects.

[dayshah]

Can you give like 2 small examples -

1. sending 2 lists of tensors that overlap
   [a, b, c], [c, d, e]

2. sending the same tensor twice

[stephanie-wang]

We might as well do the error detection in the same PR so we can provide a code sample showing what error to expect.

Also, I think this assumes too much system knowledge from the user.

Due to an issue with our implementation of memory deregistration,

-> "Due to a known issue"

repeated transfers of tensors that share the same memory space but belong to different objects.

Technically this is possible, but you have to make sure to free the first object before the second. It's probably clearer with code examples, like dhyey said.