ARROW-11667: [Rust] Add documentation for utf8 comparison kernels
ARROW-11667: [Rust] Add documentation for utf8 comparison kernels

ARROW-11667: [Rust] Add documentation for utf8 comparison kernels

When looking at the docs for https://docs.rs/arrow/3.0.0/arrow/compute/kernels/comparison/index.html I noticed there were several kernels without documentation, so I added some.

Closes #9513 from alamb/alamb/doc_comments

Authored-by: Andrew Lamb <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@nerdnetworks.org>
Signed-off-by: Andrew Lamb <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@nerdnetworks.org>

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