The kernel repository has pretty extensive documentation on how to describe the code changes in the commit messages:

- https://docs.kernel.org/process/submitting-patches.html#desc...

- https://github.com/torvalds/linux/blob/master/Documentation/...

- https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...

Git's own guidelines also have a nice description on how to write a better commit message:

- https://git-scm.com/docs/SubmittingPatches

- https://github.com/git/git/blob/master/Documentation/Submitt...

- https://git.kernel.org/pub/scm/git/git.git/tree/Documentatio...