Document HashToField security level

[kevaundray]

The OPCODE HashToField requires that the hash function heuristically behaves as a random oracle.

Kobi Gurkan pointed out that the security level should also be documented.

We can either:

• Accept a min security parameter . This would allow more flexibility if the underlying proving system can provide less than 128 bits and you do not need it.

• Document ACIR indicating that we always require 128 bit security.

[kevaundray]

This will be left open until there is a specific place to document opcode