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.

Read the fourth paragraph of the devguide's "Handling Others' Code" section and review the linked Discourse topic for context. Clarify when GitHub's Co-authored-by: line should be used for significant review or suggested-change contributions, including how it relates to squash, automerge, and direct suggested changes.