* remove content_object_init section from docs
* improve content_object_init test
The content_object_init signal used to set its class as sender and pass
the instance as additional arg in 6100773. Commit ed907b4 removed this
behaviour to bring it inline with other signals, on the basis that
you can test for the class of the object anyway.
We also had a test in place, checking this behaviour, but it was poorly
implemented, not actually checking if the function ever got called.
closes#1711
e3f5e39e17