Clarify arrow in snapshot tests

Author: Kobzol

src/bootstrap/src/core/builder/tests.rs

@@ -1273,6 +1273,10 @@ mod staging {
 /// Renders the executed bootstrap steps for usage in snapshot tests with insta.
 /// Only renders certain important steps.
 /// Each value in `steps` should be a tuple of (Step, step output).
+///
+/// The arrow in the rendered output (`X -> Y`) means `X builds Y`.
+/// This is similar to the output printed by bootstrap to stdout, but here it is
+/// generated purely for the purpose of tests.
 fn render_steps(steps: &[(Box<dyn Any>, Box<dyn Any>)]) -> String {
     steps
         .iter()