Add a note with more info about num_slashing_spans for the call withdraw_unbonded in the Staking pallet.

[IkerAlus]

It would be nice to add a note or further explanation for the documentation around the input num_slashing_spans for the call withdraw_unbonded in the Staking pallet.

Tools offering staking services have asked about what is the right value to input for withdraw_unbonded assuming they had to use an exact number of slashing spans to remove. However, as far as I understand, this input doesn't affect any state change/logic in the call, it is only a thumbs up/down on whether to proceed with the deletion of slashing spans knowing there is enough fee to pay for it. A sentence stating this point as a note would be enough to avoid future confusion.