Document source compatibility break in QAtomic classes

My patch to fix the source incompatibility was rejected: http://codereview.qt-project.org/#change,18102

At a minimum, the source incompatibility should be documented in dist/changes-5.0.0.

Additionally, regardless of my patch, the right and wrong way to use the QAtomic classes should be documented in the documentation of those classes. The commit which removed the implicit casts cites 'some concerns about misuse'. Those concerns will only ever be addressed if they are documented.