doc: remove vestigial onboarding section

The onboarding-extras doc includes a section with a single bullet point instructing the new Collaborator to write good pull request descriptions. This material is likely superfluous and is in the wrong document if it is not. (It more properly belongs in pull-requests.md as it is information that is not specific to new Collaborators.) I am not putting it in pullrequests.md because that document is already plenty long and detailed, and includes advice on writing good commit messages, which tend to become the pull request descriptions anyway.

Checklist

• documentation is changed or added
• commit message follows commit guidelines