Add JSDoc for overclock_timers

spec/frontend_integration/test_helpers/utils/overclock_timers.js

+/**
+ * This function replaces the existing `setTimeout` and `setInterval` with wrappers that
+ * discount the `ms` passed in by `boost`.
+ *
+ * For example, if a module has:
+ *
+ * ```
+ * setTimeout(cb, 100);
+ * ```
+ *
+ * But a test has:
+ *
+ * ```
+ * useOverclockTimers(25);
+ * ```
+ *
+ * Then the module's call to `setTimeout` effectively becomes:
+ *
+ * ```
+ * setTimeout(cb, 4);
+ * ```
+ *
+ * It's important to note that the timing for `setTimeout` and order of execution is non-deterministic
+ * and discounting the `ms` passed could make this very obvious and expose some underlying issues
+ * with flaky failures.
+ *
+ * WARNING: If flaky spec failures show up in a spec that is using this helper, please consider either:
+ *
+ *   - Refactoring the production code so that it's reactive to state changes, not dependent on timers.
+ *   - Removing the call to this helper from the spec.
+ *
+ * @param {Number} boost
+ */
 // eslint-disable-next-line import/prefer-default-export
 export const useOverclockTimers = (boost = 50) => {
   if (boost <= 0) {