Soundness of syscall read preimage

[dannywillems]

          Nit (not for this PR): a good documentation, IMO, is also a documentation that describes what the parameters of the function does, and in particular, explains why there are some unused parameters when it is in the implementation of a trait.

For instance, here, I would have explained why _addr and _len are unused in the constraints side, when it is used in the witness. And also explain what pos is expected to be.

Originally posted by @dannywillems in https://github.com/o1-labs/proof-systems/pull/2274#discussion_r1642929659

Also:

• [ ] https://github.com/o1-labs/proof-systems/pull/2274#discussion_r1643200022