doc: clarify why blinding factor range proof is needed

[starius]

This place in the intro was very hard to me to understand. I think now it is easier to understand the logic of why range proofs of blinding factors are needed. Hopefully it helps future readers!

[IRsoctierDT]

True

[Anynomouss]

I agree that the description could be made a bit clearer, but the proposed changes are also confusing. I would suggest to change the text as follows:

This output ((113 + 99)*G + 2*H) requires that both the numbers 113 and 99 are known in order to be spent; the attacker would thus have successfully locked Carol's UTXO. The requirement for a range proof for the blinding factor prevents this because the attacker doesn't know the number 113 and thus neither (113 + 99). In other words, without knowing the private key (blinding factor), the attacker would not know the value in the output and would not be able to produce a valid range proof for it. A more detailed description of range proofs is further detailed in the range proof paper.

[wiesche89]

PR #3840 includes @Anynomouss review.