secp256k1.h: clarify that by default arguments must be != NULL
secp256k1.h: clarify that by default arguments must be != NULL

The same file says that the illegal callback will only triger for violations
explicitly mentioned, which is not true without this commit because we often
don't mention that an argument is not allowed to be NULL.

bitcoin-core/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,ElementsProject/secp256k1-zkp,peterdettman/secp256k1,peterdettman/secp256k1,bitcoin-core/secp256k1