Improve docs for `from_dlpack` and `to_dlpack` (#70437)
Summary:
This moves the warning to the legacy function where it belongs, improves the phrasing, and adds examples.
There may be more to do to make `from_dlpack` more discoverable as a follow-up, because in multiple issues/PR we discovered people wanted new things (e.g., a memoryview-like object, or `__array_interface__` support) that `from_dlpack` already provides.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70437
Reviewed By: albanD
Differential Revision: D33760552
Pulled By: mruberry
fbshipit-source-id: e8a61fa99d42331cc4bf3adfe494cab13ca6d499
(cherry picked from commit 880ad9665956078958af93132a4c6ae820bbaac9)
