[R] Add content on using open_dataset() with schemas vs. column names

For more information, see the following discussion on a PR on the main arrow repo: https://github.com/apache/arrow/pull/12083

Start by reading the linked Apache Arrow pull request discussion and compare its details with the cookbook's existing R content. Identify where documentation for open_dataset() belongs, then add guidance distinguishing schemas from column names and verify that the new content addresses the discussion.

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