Adds missing info about changes in StrictMode related to useEffect

closes reactjs#4649

content/docs/hooks-effect.md

@@ -476,6 +476,8 @@ In the future, the second argument might get added automatically by a build-time
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 >If you want to run an effect and clean it up only once (on mount and unmount), you can pass an empty array (`[]`) as a second argument. This tells React that your effect doesn't depend on *any* values from props or state, so it never needs to re-run. This isn't handled as a special case -- it follows directly from how the dependencies array always works.
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+>**However, since React 18 in [`StrictMode`](/docs/strict-mode.html) the new check will automatically unmount and remount every component, whenever a component mounts for the first time.**. Meaning React will call the `useEffect` callbacks twice, even for the empty dependencies array. The `StrictMode` works only in the `development` environment - you will not observe this behavior in the `production` build. The reasoning behind this is described in the [React 18 upgrade guide](/blog/2022/03/08/react-18-upgrade-guide.html#updates-to-strict-mode). 
+>
 >If you pass an empty array (`[]`), the props and state inside the effect will always have their initial values. While passing `[]` as the second argument is closer to the familiar `componentDidMount` and `componentWillUnmount` mental model, there are usually [better](/docs/hooks-faq.html#is-it-safe-to-omit-functions-from-the-list-of-dependencies) [solutions](/docs/hooks-faq.html#what-can-i-do-if-my-effect-dependencies-change-too-often) to avoid re-running effects too often. Also, don't forget that React defers running `useEffect` until after the browser has painted, so doing extra work is less of a problem.
 >
 >We recommend using the [`exhaustive-deps`](https://github.com/facebook/react/issues/14920) rule as part of our [`eslint-plugin-react-hooks`](https://www.npmjs.com/package/eslint-plugin-react-hooks#installation) package. It warns when dependencies are specified incorrectly and suggests a fix.

content/docs/hooks-reference.md

@@ -188,6 +188,8 @@ Now the subscription will only be recreated when `props.source` changes.
 >
 >If you want to run an effect and clean it up only once (on mount and unmount), you can pass an empty array (`[]`) as a second argument. This tells React that your effect doesn't depend on *any* values from props or state, so it never needs to re-run. This isn't handled as a special case -- it follows directly from how the dependencies array always works.
 >
+>**However, since React 18 in [`StrictMode`](/docs/strict-mode.html) the new check will automatically unmount and remount every component, whenever a component mounts for the first time.**. Meaning React will call the `useEffect` callbacks twice, even for the empty dependencies array. The `StrictMode` works only in the `development` environment - you will not observe this behavior in the `production` build. The reasoning behind this is described in the [React 18 upgrade guide](/blog/2022/03/08/react-18-upgrade-guide.html#updates-to-strict-mode). 
+>
 >If you pass an empty array (`[]`), the props and state inside the effect will always have their initial values. While passing `[]` as the second argument is closer to the familiar `componentDidMount` and `componentWillUnmount` mental model, there are usually [better](/docs/hooks-faq.html#is-it-safe-to-omit-functions-from-the-list-of-dependencies) [solutions](/docs/hooks-faq.html#what-can-i-do-if-my-effect-dependencies-change-too-often) to avoid re-running effects too often. Also, don't forget that React defers running `useEffect` until after the browser has painted, so doing extra work is less of a problem.
 >
 >