Overriding signals is no longer possible in QQ2. Document that.

Resurrect commit 6f411ef9d460d6a4f73a455b6eec9afc2f52d305
in declarative. This got lost since the docs were moved.

Original commit by Sérgio Martins <sergio.martins.qnx@kdab.com>

Change-Id: I8deab99a02e09c380bd0292226f2b3658e9b321b
Reviewed-by: Sérgio Martins <sergio.martins.qnx@kdab.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>