Kicchu,
There is no problem to insert scan on flip flop with reset pins. You just need to control your reset path that's why no flip flop on reset path must be scanable.
What your books want to say, you don't have scan flip flop which control synchronized reset or clock gating.
For example, for a simple stuck at fault test, there are three steps :
1- shift-in :Each input scanable Flip Flop is set in a state through the different scan chain.
2 - capture : Flip flop captures their input.
3 - shift out : you can obsereve the result
If a flip flop which controls a reset is in a scan chain,the reset is asserted during the second step. Then some scanable flip flop can be reset and they don't capture their input but set their ouput to zero.
This is the same for clock gating. Imagine a flop which enable a clock gating, it means capture is not possible for certain flops.