Fixed constructor -> componentDidMount in docs

.changeset/flat-pumas-cross.md

@@ -1,12 +1,8 @@
 ---
 "mobx-react-lite": patch
+"mobx-react": major
 ---
 
 -   Fixed `observer` in `StrictMode` #3671
-
----
-
-## "mobx-react": major
-
--   Class component's `props`/`state`/`context` are no longer observable. Attempt to use these in any derivation other than component's `render` throws and error.
+-   **[BREAKING CHANGE]** Class component's `props`/`state`/`context` are no longer observable. Attempt to use these in any derivation other than component's `render` throws and error. For details see https://github.com/mobxjs/mobx/blob/main/packages/mobx-react/README.md#note-on-using-props-and-state-in-derivations
 -   Extending or applying `observer` classes is now explicitly forbidden

packages/mobx-react/README.md

@@ -122,7 +122,9 @@ class Alerter extends React.Component<{ counter: number }> {
 
     constructor(props) {
         this.observableCounter = counter
+    }
 
+    componentDidMount() {
         // set up a reaction, by observing the observable,
         // rather than the prop which is non-reactive:
         this.reactionDisposer = autorun(() => {
@@ -137,13 +139,13 @@ class Alerter extends React.Component<{ counter: number }> {
         this.observableCounter = this.props.counter
     }
 
-    render() {
-        return <div>{this.props.counter}</div>
-    }
-
     componentWillUnmount() {
         this.reactionDisposer()
     }
+
+    render() {
+        return <div>{this.props.counter}</div>
+    }
 }
 ```
 

packages/mobx-react/src/observerClass.ts

@@ -252,7 +252,7 @@ function createObservablePropDescriptor(key: "props" | "state" | "context") {
                 throw new Error(
                     `[mobx-react] Cannot read "${admin.name}.${key}" in a reactive context, as it isn't observable.
                     Please use component lifecycle method to copy the value into a local observable first.
-                    See TODO doc link`
+                    See https://github.com/mobxjs/mobx/blob/main/packages/mobx-react/README.md#note-on-using-props-and-state-in-derivations`
                 )
             }
             return admin[key]