Clarify how updating pull requests works

Closes #374.

Author: rmzelle

book/06-github/sections/2-contributing.asc

@@ -153,9 +153,12 @@ Anyone can also leave general comments on the Pull Request. In <<_pr_discussion>
 .Pull Request discussion page
 image::images/blink-05-general-comment.png[PR discussion page]
 
-Now the contributor can see what they need to do in order to get their change accepted. Luckily this is also a very simple thing to do. Where over email you may have to re-roll your series and resubmit it to the mailing list, with GitHub you simply commit to the topic branch again and push.
+Now the contributor can see what they need to do in order to get their change accepted.
+Luckily this is very straightforward.
+Where over email you may have to re-roll your series and resubmit it to the mailing list, with GitHub you simply commit to the topic branch again and push, which will automatically update the Pull Request.
+In <<_pr_final>> you can also see that the old code comment has been collapsed in the updated Pull Request, since it was made on a line that has since been changed.
 
-If the contributor does that then the project owner will get notified again and when they visit the page they will see that it's been addressed. In fact, since a line of code changed that had a comment on it, GitHub notices that and collapses the outdated diff.
+Adding commits to an existing Pull Request doesn't trigger a notification, so once Tony has pushed his corrections he decides to leave a comment to inform the project owner that he made the requested change.
 
 [[_pr_final]]
 .Pull Request final