Reword branches chapter

The following are some improvements for the branches chapter that I've unfortunately left sitting for a long time. 
I wasn't keen to go further without knowing where @rthorp was heading since my previous two PRs were my first public contributions ever. Now there's a burst of activity getting the documentation ready for a new release, so I hope you might have time to consider these changes. I've just rebased on "latest" and squashed them into one commit.