Developer guide edits: rebase, cleanup, PRs, format

[dagood]

Formatting:

• Title -> sentence case formatting from Add making changes section to DeveloperGuide #1417
• Add linebreaks for better reviewability and consistency.

A few changes:

• Mention git go-patch rebase.
• Be more specific about cleanup--under what circumstances there's a risk if you don't clean up.
• I don't think some of the text about patch files is necessary: patch files are explained early in the doc.
• Add more info about PRs.

[qmuntal]

Approved with some minor comments.