docs(CHANGELOG.md): add breaking change from 5f3f25a1

5f3f25a1 included a breaking change which was not documented, which is that
the return value of directive constructors is ignored. The reason they are
ignored is to ensure that the correct object is bound to when binding
properties to the controller. It may be possible to come up with a better
solution which informs the developer that what they are doing is wrong, rather
than just breaking instead.

Closes #8876