Add explanation about composeStepRunners

code/lib/store/src/csf/stepRunners.ts

@@ -1,5 +1,23 @@
 import { AnyFramework, StepRunner } from '@storybook/csf';
 
+/**
+ * Compose step runners to create a single step runner that applies each step runner in order.
+ *
+ * A step runner is a a function that takes a defined step: `step('label', () => { ... })`
+ * and runs it. The prototypical example is from `@storybook/addon-interactions` where the
+ * step runner will decorate all instrumented code inside the step with information about the
+ * label.
+ *
+ * In theory it is possible to have more than one addon that wants to run steps; they can be
+ * composed together in a similar fashion to decorators. In some ways step runners are like
+ * decorators except it is not intended that they change the context or the play function.
+ *
+ * The basic implementation of a step runner is `async (label, play, context) => play(context)`
+ *  -- in fact this is what `composeStepRunners([])` will do.
+ *
+ * @param stepRunners an array of StepRunner
+ * @returns a StepRunner that is the composition of the arguments
+ */
 export function composeStepRunners<TFramework extends AnyFramework>(
   stepRunners: StepRunner<TFramework>[]
 ): StepRunner<TFramework> {