How to determine a corner solution? Growth Model
What the authors seem to be doing in the footnote is imposing a condition stating that if intergenerational transfers tend to infinity (say for the case of children of billionaires) there will be no growth in wealth from human capital investment.
This condition is here to just communicate that the parent views allocation between human capital of the child and physical capital as substitutes.
This is communacated by the fact that we have $w_{t+1}h_{t+1}+r_{t+1}s_{t+1}$ being concidered in the same logarithm together.
Hope this helps