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ARROW-9695: [Rust] Improve comments on LogicalPlan enum variants

While reviewing the DataFusion code more (thanks again for this great piece of work) I have some small suggestions on improving the documentation that I would have personally appreciated while reading the code. I wanted to contribute them back

Closes #7934 from alamb/alamb/improve-comments

Authored-by: alamb <andrew@nerdnetworks.org>
Signed-off-by: Andy Grove <andygrove73@gmail.com>