fix: VerifyProof comment to reflect in-circuit behavior

[Snezhkko]

• Update VerifyProof comment: no boolean return, no numLeaves/nil checks.
• Clarify that leaf is the leaf index (little-endian bits) and actual leaf value is mp.Path[0].
• Explain that verification is encoded via constraints asserting recomputed root equals mp.RootHash.

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Note

Update VerifyProof doc to clarify constraint-based verification, leaf index bit order, leaf value in mp.Path[0], and no return value.

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[ivokub]

Thanks for the contribution. Indeed the change helps the documentation a bit.