alt_bn128_pairing_product's return type is incorrectly documented

[anton-trunov]

The function applies a pairing function on each point to check for equality and returns True or False depending on whether the pairing check succeeds or fails.

This implies alt_bn128_pairing_product's result type is Bool, however it actually is Option Bool. The builtin returns None if the inputs are invalid (can this actually happen in a well-typed program?).