Plonkup - Document proof widgets evaluation constants

[vlopes11]

In modularise_plookup, a function ProverKey::num_multiset was introduced with constant value 4

However, this is not documented anywhere and could create some future confusion.

[moCello]

File: src/proof_system/widget.rs

 273   │         /// Returns the number of [`MultiSet`]s contained in a ProverKey.
 274   │         const fn num_multiset() -> usize {
 275   │             // FIXME https://github.com/dusk-network/plonk/issues/581
 276   │             4
 277   │         }