minor fixes

1-js/07-object-properties/01-property-descriptors/article.md

@@ -194,7 +194,7 @@ alert(Object.keys(user)); // name
 
 The non-configurable flag (`configurable:false`) is sometimes preset for built-in objects and properties.
 
-A non-configurable property can not be deleted.
+A non-configurable property can't be deleted, its attributes can't be modified.
 
 For instance, `Math.PI` is non-writable, non-enumerable and non-configurable:
 
@@ -214,14 +214,23 @@ alert( JSON.stringify(descriptor, null, 2 ) );
 So, a programmer is unable to change the value of `Math.PI` or overwrite it.
 
 ```js run
-Math.PI = 3; // Error
+Math.PI = 3; // Error, because it has writable: false
 
 // delete Math.PI won't work either
 ```
 
+We also can't change `Math.PI` to be `writable` again:
+
+```js run
+// Error, because of configurable: false
+Object.defineProperty(Math, "PI", { writable: true });
+```
+
+There's absolutely nothing we can do with `Math.PI`.
+
 Making a property non-configurable is a one-way road. We cannot change it back with `defineProperty`.
 
-**The idea of "configurable: false" is to prevent changes of property flags and its deletion, while allowing to change its value.**
+**Please note: `configurable: false` prevents changes of property flags and its deletion, while allowing to change its value.**
 
 Here `user.name` is non-configurable, but we can still change it (as it's writable):
 
@@ -238,7 +247,7 @@ user.name = "Pete"; // works fine
 delete user.name; // Error
 ```
 
-And here we make `user.name` a "forever sealed" constant:
+And here we make `user.name` a "forever sealed" constant, just like the built-in `Math.PI`:
 
 ```js run
 let user = {
@@ -258,9 +267,9 @@ Object.defineProperty(user, "name", { value: "Pete" });
 ```
 
 ```smart header="The only attribute change possible: writable true -> false"
-There's a minor exception, that we have to mention.
+There's a minor exception about changing flags.
 
-We can change `writable: true` to `false` for a non-configurable property, thus making the property readonly (can add another layer of protection). But not the other way around.
+We can change `writable: true` to `false` for a non-configurable property, thus preventing its value modification (to add another layer of protection). Not the other way around though.
 ```
 
 ## Object.defineProperties