step: backfill some documentation

crates/cross/src/step/mod.rs

@@ -12,6 +12,7 @@ pub use support::{
     VolumeSamplePos,
 };
 
+// TODO: make the fields private and hide behind a constructor?
 pub struct StepEContext<'a, R, M> {
     pub inv_feature_size: R,
     pub time_step: R,
@@ -23,6 +24,9 @@ pub struct StepEContext<'a, R, M> {
 }
 
 impl<'a, R: Real, M: Material<R>> StepEContext<'a, R, M> {
+    /// given the simulation resoures (material, stimulus, fields),
+    /// advance the `e` field at the provided index.
+    /// this is a good toplevel function to use if you don't care about any of the details.
     pub fn step_flat_view<RM, RF, WF>(
         meta: SimMeta<R>,
         mat: &RM,
@@ -59,6 +63,7 @@ impl<'a, R: Real, M: Material<R>> StepEContext<'a, R, M> {
         e_matrix[idx] = new_e;
     }
 
+    /// return the e field for this cell at `t=t_{now} + \delta t`
     pub fn step_e(self) -> Vec3<R> {
         let twice_eps0 = R::twice_eps0();
         let deltas = self.in_h.delta_h();
@@ -90,6 +95,9 @@ pub struct StepHContext<'a, R, M> {
 }
 
 impl<'a, R: Real, M: Material<R>> StepHContext<'a, R, M> {
+    /// given the simulation resoures (material, stimulus, fields),
+    /// advance the `h` field at the provided index.
+    /// this is a good toplevel function to use if you don't care about any of the details.
     pub fn step_flat_view<RM, RF, WF>(
         meta: SimMeta<R>,
         mat: &RM,
@@ -131,6 +139,7 @@ impl<'a, R: Real, M: Material<R>> StepHContext<'a, R, M> {
         m_matrix[idx] = new_m;
     }
 
+    /// return the `(h, m)` fields for this cell at `t=t_{now} + \delta t`
     pub fn step_h(self) -> (Vec3<R>, Vec3<R>) {
         let mu0 = R::mu0();
         let mu0_inv = R::mu0_inv();