message: Add comments describing `InputFieldIngredient`
diff: @@ -7,6 +7,11 @@ use crate::key::DependencyIndex;
  use crate::runtime::local_state::QueryInputs;
  use crate::runtime::StampedValue;
  +/// Ingredient used to represent the fields of a `#[salsa::input]`.
  +/// These fields can only be mutated by an explicit call to a setter
  +/// with an `&mut` reference to the database,
  +/// and therefore cannot be mutated during a tracked function or in parallel.
  +/// This makes the implementation considerably simpler.
  pub struct InputFieldIngredient<K, F> {
  index: IngredientIndex,
  map: FxHashMap<K, StampedValue<F>>
  