reproject_exact missing method description or reference

The docs `reproject_exact` (https://reproject.readthedocs.io/en/stable/api/reproject.reproject_exact.html#reproject.reproject_exact) are currently missing an explanation of how the method works, or a reference to somewhere else that explains how the method works.

Start with the reproject_exact API documentation at the linked Read the Docs URL and inspect the surrounding documentation to find where its method description is defined. Add an explanation of how reproject_exact works or a reference to an existing explanation, then verify the API page presents that information.

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