r200132 - Enforce safe usage of DiagnosticsEngine::getCustomDiagID()

[Rafael Espíndola]

> Docstrings updated in r200137.
>
> It the builders are enforcing -Wdocumentation it should really also be
> enabled in the default CMake configuration.

I think we used to have on build bot using it.

I noticed because I find it useful and have it enabled locally. So I
agree, it would be nice have cmake enable it by default :-)

Cheers,
Rafael