Doc: update Source Breaks topic

With important behavior changes from the change logs.
Fix some formatting and language issues.
Add subheadings to make it easier to find changes to
individual classes.
Where there are more than two changes to a class,
group them under a subheading.

Task-number: QTBUG-37811
Change-Id: I0b0fd56bc42bcb6f72b1c61a24865b99e1e2edad
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>