The placement differences are partially explained in the manual. The closed expansion tank functions as an air eliminator, which is why it must be mounted above the boiler. That is the tank must be located so that any gases which come out of solution inside the boiler migrate to the tank.
The diaphragm tank requires air to be removed through an automatic vent. In that case the entire system is designed to run without air in it, which is why the system is plumbed to pump out of the tank. To put it simply, the tank is a point of no pressure change. Gory details here.
Basically the circulator creates a change in pressure while it's running. If it discharges into the tank the pressure is subtracted, because the point of no pressure change is at the discharge. The vacuum caused by the pump can then lower the water level enough to allow flashing to steam or drawing air in through vents.
Conversely, if the circulator draws from the tank the pump suction will remain constant whether the pump is running or not. When the pump is running the discharge pressure will increase causing a gradient of increased pressure in the direction of flow around the loop, which is a favorable condition.
There are practical engineering limits at play. Obviously the "relative badness" of improperly locating the tank varies with the size of the pump and pressure of the system. I really have no idea how critical the issue is in your case. Then again, the tank need not be physically located below the circulator to be plumbed in before it. Excluding ludicrous scenarios such as putting it forty stories above the boiler you can locate it someplace convenient and pipe over to it.
Do keep in mind that I'm not a licensed plumber or HVAC tech so I wouldn't consider this coming from a "credible and/or official source." Do feel free to double check with a qualified professional.