Why additional input series damping for AD797
Remember that to excite a resonant mode you have to put energy into that node at the right frequency.  What they are saying is that for low noise reasons they made the parasitic resistance on the order of 6 Ohms. The lower that either R or C is in the RC time constant the higher the frequency that is passed.  So what they are saying is that putting a 100 Ohm resistor in there will take the edges off just enough that the risk of the device ringing is greatly reduced.  Of course there are considerations for noise and bandwidth so this value is as small as possible while still doing little to add to noise.