ARROW-10160: [Rust] Improve DictionaryType documentation (clarify which type is which)
ARROW-10160: [Rust] Improve DictionaryType documentation (clarify which type is which)

I kept having to look this up, so I figured I would update the docs about it

Closes #8327 from alamb/alamb/ARROW-10160-docs

Authored-by: alamb <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@nerdnetworks.org>
Signed-off-by: Paddy Horan <e3329cc3e496dc07f33f8b36f2d1ccb51669f92b@hotmail.com>

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