[C++] -Wdocumentation failures with clang 6.0 on Ubuntu 18.04

[asfimport]

I guess these aren't caught on Ubuntu 14.04. Patch incoming

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #3310

_{Note: This issue was originally created as ARROW-4157. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 3310
#3310