docs: clarify Strict Mode and Fast Refresh effects

docs/fast-refresh.md

@@ -45,3 +45,10 @@ Hooks with dependencies—such as `useEffect`, `useMemo`, and `useCallback`—wi
 For example, when you edit `useMemo(() => x * 2, [x])` to `useMemo(() => x * 10, [x])`, it will re-run even though `x` (the dependency) has not changed. If React didn't do that, your edit wouldn't reflect on the screen!
 
 Sometimes, this can lead to unexpected results. For example, even a `useEffect` with an empty array of dependencies would still re-run once during Fast Refresh. However, writing code resilient to an occasional re-running of `useEffect` is a good practice even without Fast Refresh. This makes it easier for you to later introduce new dependencies to it.
+
+Effects may also run again during development for reasons unrelated to Fast Refresh:
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+- Fast Refresh re-runs Hooks after a code edit, ignoring dependency lists so the changes take effect.
+- When [Strict Mode](https://react.dev/reference/react/StrictMode#fixing-bugs-found-by-re-running-effects-in-development) is enabled at the application root, React runs an additional setup and cleanup cycle when a component first mounts to help find missing cleanup.
+
+In both cases, Effects should implement complete cleanup and tolerate being restarted.