ARROW-12000: [Documentation] Add note about deviation from style guide on struct/classes
ARROW-12000: [Documentation] Add note about deviation from style guide on struct/classes

![docs](https://user-images.githubusercontent.com/1696093/111465419-2b1e8880-86c6-11eb-98d5-5e5b873c224c.png)

Closes #9739 from westonpace/feature/arrow-12000

Authored-by: Weston Pace <0dd0706858a011bb30a3fd82b71cee4f9094c502@gmail.com>
Signed-off-by: David Li <dc24f9cfc84fb1675ce65c8444a2c3e2f0e5f297@gmail.com>

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