Docs: Document "Storage interface" at QUnit.config.storage

Storing failures and looking them up during rerun, requires only get,
set, and remove.

But, to clean up unknown past-failures after a success, we also use
`key()` and `length` to find any key that matches our naming scheme.

docs/api/config/storage.md

@@ -25,4 +25,47 @@ The Storage object to use for remembering failed tests between runs.
 
 This is used to power the [reorder feature](./reorder.md). In [browser environments](../../browser.md) this will use `sessionStorage` if supported by the browser.
 
-In Node.js and other non-browser environments, there is no storage object available for this purpose by default. You can attach your own preferred form of persistence between test runs, by assigning an object to `QUnit.config.storage` that implements `getItem`, `setItem` and `removeItem` methods, similar to the [Web Storage API](https://developer.mozilla.org/en-US/docs/Web/API/Web_Storage_API).
+In Node.js and other non-browser environments, there is no storage object available for this purpose by default. You can attach your own preferred form of persistence between test runs, by assigning an object to `QUnit.config.storage` that implements at least the below subset of the [Web Storage API](https://developer.mozilla.org/en-US/docs/Web/API/Web_Storage_API).
+
+## Storage interface
+
+```js
+storage = {
+  /**
+   * @param {string} key
+   * @param {string} value
+   */
+  setItem (key, value) {
+  },
+
+  /**
+   * @param {string} key
+   * @return {string|null}
+   */
+  getItem (key) {
+  },
+
+  /**
+   * @param {string} key
+   */
+  removeItem (key) {
+  },
+
+  /**
+   * Get name of key at given offset, e.g. by iterating from 0 to `length`.
+   *
+   * @param {number} index
+   * @return {string|null}
+   */
+  key (index) {
+  },
+
+  /**
+   * How many keys exist.
+   *
+   * @type {number}
+   */
+  get length () {
+  }
+};
+```