PSR-2 "Multi-line Arguments" Errata

Now that the [Errata Vote](https://groups.google.com/forum/?fromgroups=#!topic/php-fig/qTROKw07848) has passed we are able to add Errata to PSRs. This pull request adds the meta document, along with the first clarification I'd like to make: [Multi-line Arguments](https://gist.github.com/philsturgeon/6320152#file-multi-line-arguments-md).