Wikipedia, which is probably sufficiently accurate for these purposes, wrote:
A parking pawl is a device fitted to a car's automatic transmission that locks up the transmission. It is engaged when the shift selector is placed in the Park position, which is always the first position (topmost on a column shift, frontmost on a floor shift) in all cars sold in the United States since 1965 (when the order was standardised by the SAE) and in most other vehicles worldwide.
The parking pawl locks the transmission's output shaft to the transmission casing by engaging a pawl (a pin) that engages in a notched wheel on the shaft, stopping it (and thus the driven wheels) from turning.
Most manufacturers and mechanics do not recommend using the transmission's parking pawl as the sole means of securing a parked vehicle, instead recommending it should be engaged after first applying the vehicle's parking brake. Constant use of the parking pawl only, especially when parking on a steep incline, means that driveline components are kept constantly under stress, and can cause wear and eventual failure of the parking pawl or transmission linkage. The pawl might also fail or break if the vehicle is pushed with sufficient force, if the parking brake is not firmly engaged. Replacement can be an expensive operation since it generally requires removing the transmission from the car.
It highly inadvisable to use the parking pawl to stop a vehicle in motion. The pawl mechanism is not strong enough to stop a vehicle in motion or may not engage at all. Under that much stress, the pawl may break off in the transmission, leading to costly repairs.
Whether or not I park on an incline, I point the front wheels so that, whether forwards or backwards, the car will not roll into traffic.
Additionally, if there is a curb (kerb?), I turn my wheels toward it, which would also prevent the car from rolling into traffic, while also satisfying the previous condition above.
With either a front tire, or a front and rear tire both in contact with the curb, effectively holding the car in place, I then set the cable brake, and then put the shifter in park. Rubber and concrete handle such stresses better than brake shoes and drums, which in turn do better than the parking pawl.
Cable brakes and parking pawls are not meant to stop a car, they're meant to keep the car still.
(Yes, I know that the cable brake is often referred to as an emergency brake because it might be able to stop the car, but that isn't it's primary purpose, and as Paige well knows, it might not stop the car.)
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I think the advice Vauxhall gave out re the dodgy handbrake on Vectras also advise this. It ist also in the Highway Code too,. (Rule 252 ... ) 
