Add deprecation guides for Function.prototype.{on,property,observes}.

content/ember/v3/observes-function-prototype-extension.md

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+---
+id: function-prototype-extensions.observes
+title: Function.prototype.observes
+until: '4.0.0'
+since: '3.11'
+---
+
+Historically, Ember has extended the `Function.prototype` with a few functions
+(`on`, `observes`, `property`), over time we have moved away from using these
+prototype extended functions in favor of using the official ES modules based
+API.
+
+In order to migrate away from `Function.prototype.observes` you would update to using
+`observer` from `@ember/object` ([see
+documentation](https://api.emberjs.com/ember/release/functions/@ember%2Fobject/observer))
+directly.
+
+For example, you would migrate from:
+
+```js
+import EmberObject from '@ember/object';
+
+export default EmberObject.extend({
+  valueObserver: function() {
+    // Executes whenever the "value" property changes
+  }.observes('value')
+});
+```
+
+Into:
+
+```js
+import EmberObject, { observer } from '@ember/object';
+
+export default EmberObject.extend({
+  valueObserver: observer('value', function() {
+    // Executes whenever the "value" property changes
+  })
+});
+```
+
+Please review the deprecation RFC over at
+[emberjs/rfcs](https://emberjs.github.io/rfcs/0272-deprecation-native-function-prototype-extensions.html)
+for more details.

content/ember/v3/on-function-prototype-extension.md

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+---
+id: function-prototype-extensions.on
+title: Function.prototype.on
+until: '4.0.0'
+since: '3.11'
+---
+
+Historically, Ember has extended the `Function.prototype` with a few functions
+(`on`, `observes`, `property`), over time we have moved away from using these
+prototype extended functions in favor of using the official ES modules based
+API.
+
+In order to migrate away from `Function.prototype.on` you would update to using
+`@ember/object/evented` ([see
+documentation](https://api.emberjs.com/ember/release/functions/@ember%2Fobject%2Fevented/on))
+directly.
+
+For example, you would migrate from:
+
+```js
+import EmberObject from '@ember/object';
+import { sendEvent } from '@ember/object/events';
+
+let Job = EmberObject.extend({
+  logCompleted: function() {
+    console.log('Job completed!');
+  }.on('on')
+});
+
+let job = Job.create();
+
+sendEvent(job, 'completed'); // Logs 'Job completed!'
+```
+
+Into:
+
+```js
+import EmberObject from '@ember/object';
+import { on } from '@ember/object/evented';
+import { sendEvent } from '@ember/object/events';
+
+let Job = EmberObject.extend({
+  logCompleted: on('completed', function() {
+    console.log('Job completed!');
+  })
+});
+
+let job = Job.create();
+
+sendEvent(job, 'completed'); // Logs 'Job completed!'
+```
+
+Please review the deprecation RFC over at
+[emberjs/rfcs](https://emberjs.github.io/rfcs/0272-deprecation-native-function-prototype-extensions.html)
+for more details.

content/ember/v3/property-function-prototype-extension.md

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+---
+id: function-prototype-extensions.property
+title: Function.prototype.property
+until: '4.0.0'
+since: '3.11'
+---
+
+Historically, Ember has extended the `Function.prototype` with a few functions
+(`on`, `observes`, `property`), over time we have moved away from using these
+prototype extended functions in favor of using the official ES modules based
+API.
+
+In order to migrate away from `Function.prototype.property` you would update to using
+`computed` from `@ember/object` ([see
+documentation](https://api.emberjs.com/ember/release/functions/@ember%2Fobject/computed))
+directly.
+
+For example, you would migrate from:
+
+```js
+import EmberObject from '@ember/object';
+
+let Person = EmberObject.extend({
+  init() {
+    this._super(...arguments);
+
+    this.firstName = 'Betty';
+    this.lastName = 'Jones';
+  },
+
+  fullName: function() {
+    return `${this.firstName} ${this.lastName}`;
+  }.computed('firstName', 'lastName')
+});
+
+let client = Person.create();
+
+client.get('fullName'); // 'Betty Jones'
+
+client.set('lastName', 'Fuller');
+client.get('fullName'); // 'Betty Fuller'
+```
+
+Into:
+
+```js
+import EmberObject, { computed } from '@ember/object';
+
+let Person = EmberObject.extend({
+  init() {
+    this._super(...arguments);
+
+    this.firstName = 'Betty';
+    this.lastName = 'Jones';
+  },
+
+  fullName: computed('firstName', 'lastName', function() {
+    return `${this.firstName} ${this.lastName}`;
+  })
+});
+
+let client = Person.create();
+
+client.get('fullName'); // 'Betty Jones'
+
+client.set('lastName', 'Fuller');
+client.get('fullName'); // 'Betty Fuller'
+```
+
+Please review the deprecation RFC over at
+[emberjs/rfcs](https://emberjs.github.io/rfcs/0272-deprecation-native-function-prototype-extensions.html)
+for more details.