Soft-deprecate obscure feature on property getters returning pointers

moc actually generates the right code for getters returning a pointer
to the type in question, or a reference to the type (a reference, one
would assume, does not require code changes). However, the same
extension is not valid for the setter: it can't receive the new value
by pointer.

Therefore, let's soft-deprecate the feature by removing its existence
from the documentation.

Task-number: QTBUG-33091
Change-Id: I27844213e051ec7fafeb4744089a0653aea6f1f3
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>