Funny thing....the springs are connected to the axles and all the weight of trailer is carried by them axles plus all the bouncing up and down is done right where them springs attache to them axles.........
Place the jack under the axle were the springs are attached.
The 'warning' is so some yokel doesn't place the jack under the axle tubes out in the middle, jack his rig up, bend the axles tubes and then complain to MFG. about worn out tires etc.
Yes I am one of those Internet Bozos that WILL give you advice...based on some common sense, life events, work history & practical experience.
You just have to decide whether or not to take the advise/suggestions.