[C++] Outdated and confusing comment on dictionary indices

[asfimport]

While working on the Rust implementation, I got confused by this comment that said dictionary index types could be assumed to be signed integers. The Rust code allowed for unsigned integers, so I thought the Rust code had a bug. However, I read the C++ code more carefully, and unsigned integers are allowed as dictionary index types-- this was changed in this commit but the comment wasn't updated.

I have a pull request fixing the comment, just want to save someone a few minutes of confusion :)

Reporter: Carol Nichols / @carols10cents
Assignee: Carol Nichols / @carols10cents

PRs and other links:

• GitHub Pull Request #8535

_{Note: This issue was originally created as ARROW-10397. Please see the migration documentation for further details.}

[asfimport]

Micah Kornfield / @emkornfield:
Issue resolved by pull request 8535
#8535