Update Devguide about pull request commit documentation.

[louisom]

I think the documentation of pull request commit should be improved, the current version with 3. Subitting isn't that clear for people who were new to GitHub or Python CPython development process.

[ezio-melotti]

This is a duplicate of #103, but since you have a PR I'll leave it open.

The PR looks good -- I'm going to review it in detail and merge it tomorrow.

[louisom]

#140 Close this.