[Documentation] Add note about deviation from style guide on struct/classes

[asfimport]

Per mailing list discussion there are places where we use structs that wouldn't technically be allowed by the Google style guide.  We should add a small note to our development guidelines stating as much.

Reporter: Weston Pace / @westonpace
Assignee: Weston Pace / @westonpace

PRs and other links:

• GitHub Pull Request #9739

_{Note: This issue was originally created as ARROW-12000. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm:
Issue resolved by pull request 9739
#9739