fix: clarify KoalaBear modulus bit constraints

[Fibonacci747]

Clarified the documentation for the KoalaBear modulus range checks to explicitly describe the LE bit order and which indices are constrained in less_than_bb_modulus. Also corrected the high-level explanation of the “top 7 / bottom 24 bits” logic in the circuit builder so it matches the actual inequality x < 2^31 - 2^24 + 1 instead of making an imprecise claim about the value range. This makes the intent of the constraints clear and avoids future confusion when reasoning about bit layouts for KoalaBear elements.