The Welcome template has this excellent write-up on how to make your pull request:

Please make sure that either:

• Your Pull Request has a good description, if it consists of multiple commits, as it will be used as cover letter.
• Your Pull Request description is empty, if it consists of a single commit, as the commit message should be descriptive enough by itself.

However, we often see so many single-patch submissions on the git@vger.kernel.org mailing list with a copy of the log message under the three-dash line, which is a sign that the latter point is not honored.

It should be fairly simple to auto-detect. If a pull-request consists of a single-commit branch, after removing Cc: lines, the pull request description should be empty, or you'd get a "did you really mean it? Here is what we recommend in Welcome.md file" reminder, perhaps?