Doc: Reintroduce \beginqdoc and \endqdoc macros

These macros were previously defined for QDoc manual.
This change adds them to global macros.qdocconf so
they can be used everywhere.

Change-Id: I4fdf5a103ecfb356719951a7d0c6fbd35c887744
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Correct type mismatches sentence in signals and slots documentation.

Task-number: QTBUG-15994

Change-Id: I19581ae33313de44bcb6e5eb2c06fc9e507ad101
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>