Doc comments for the functions this PR touches

crates/engine/src/tests/track.rs

@@ -440,6 +440,7 @@ fn analyze_1080p_single_point_speed() {
 struct SampleFootage;
 
 impl crate::render::ExprHost for SampleFootage {
+    /// Reads the red channel of the layer's footage pixel at (0, 0) through the context's footage source, scaled to 0-100.
     fn eval(
         &self,
         ctx: &EvalCtx,

crates/script/src/model.rs

@@ -510,6 +510,7 @@ fn text_json(d: &TextDoc) -> J {
     doc
 }
 
+/// Returns the property's value at the requested time, evaluating its expression unless `pre` is set.
 fn value(s: &Session, a: &J) -> R {
     let (cid, c, l) = layer_of(s, a)?;
     let uid = u(a, "uid").ok_or("missing uid")?;