Correct misleading comment about the choice of polynomial

[intoverflow]

Also adds a unit test to pin-down the choice of field extension.

[jacobdweightman]

There are also a few (duplicated) comments which mention X^4 - 11 in zirgen. I'm fine with this as-is, but I would also like to see those fixed.

[pdg744]

subtle nit:

I noticed that in addition to changing - 11 to + 11, this PR also changes - BETA to + BETA. It looks like BETA appears in code, and it's not quite clear to me whether we're using:

f(x) = x - BETA, where BETA = -11

f(x) = x + BETA where BETA = 11`.

@jacobdweightman @jbruestle maybe one of you can clarify?

[pdg744]

i.e. see BETA vs. NBETA here

risc0/risc0/zkp/src/field/baby_bear.rs

Line 491 in c15712d

const BETA: Elem = Elem::new(11);

[pdg744]

I think we can ignore my nit above: I realized after writing this that "NBETA" almost certainly means "negative BETA," which seems to clarify that BETA = 11 and the polynomial is f(x) = x + BETA.