[DOC] improve the documentation

closes #340
closes #344
closes #352
closes #355

doc/observer.md

@@ -1,10 +1,16 @@
 # 🦉 Observer 🦉
 
 Owl needs to be able to react to state changes. For example, whenever the state
-of a component is changed, we need to rerender it. To help with that, we have
-an Observer class. Its job is to observe some object state, and react to any
-change. To do that, it recursively replaces all keys of the observed state by
-getters and setters.
+of a component is changed, Owl needs to rerender it. To help with that, there is
+an Observer class. Its job is to observe the state of an object (or array), and
+to react to any change. The observer is implemented with the native `Proxy`
+object. Note that this means that it will not work on older browsers.
+
+Note that the `Observer` is used by the `useState` and `useContext` hooks. This
+is the way most Owl applications will create observers. For the majority of
+use cases, there is no need to directly instantiate an observer.
+
+## Example
 
 For example, this code will display `update` in the console:
 
@@ -16,5 +22,31 @@ const obj = observer.observe({ a: { b: 1 } });
 obj.a.b = 2;
 ```
 
-The observer is implemented with the native `Proxy` object. Note that this
-means that it will not work on older browsers.
+This example shows that an observer can observe nested properties.
+
+## Reference
+
+**observe** An observer can observe multiple values with the `observe` method.
+This method takes an object or an array as its argument and will return a proxy
+(which is mapped to the initial object/array). With this proxy, the observer
+can detect whenever any internal value is changed.
+
+**Registering a callback** Whenever an observer sees a state change, it will
+call its `notifyCB` method. No additional information is given to the callback.
+
+**deepRevNumber** Each observed value has an internal revision number, which
+is incremented every time the value is observed. Sometimes, it can be useful
+to obtain that number:
+
+```js
+const observer = new owl.Observer();
+const obj = observer.observe({ a: { b: 1 } });
+
+observer.deepRevNumber(obj.a); // 1
+obj.a.b = 2;
+
+observer.deepRevNumber(obj.a); // 2
+```
+
+The `deepRevNumber` can also return 0, which indicates that the value is not
+observed.