Clarify "Co-authored-by" usage in PRs

As brought up in a Discourse committers topic, the devguide could be more clear about Pull Request attribution on GitHub.

Specifically, in the fourth paragraph of "Handling Others' Code", the Co-authored-by: feature of GitHub is mentioned, but the use case of it is not defined.

Since GitHub will automatically attribute the primary author of the PR when using "squash and merge" or the "automerge", the Co-authored-by: is useful to add just prior to merging for anyone that significantly contributed to the PR, either via review or direct suggested changes. When using the GitHub UI to directly commit a suggested change to the PR, a Co-authored-by: line is automatically added to the commit message. Otherwise though, it has to be done manually for people other than the author to be attributed for contributing to the PR.

With this being a rather substantial part of the workflow for GitHub PRs, it seems to be worthwhile to mention when Co-authored-by: should be used rather than mentioning the feature without elaborating further.

devguide の「Handling Others' Code」セクションの第 4 段落を読み、コンテキストを把握するためにリンク先の Discourse トピックを確認してください。重要なレビューや提案変更による貢献に対して GitHub の Co-authored-by: 行を使用すべきタイミングを明確にし、squash、automerge、直接の提案変更との関係も含めてください。