Move the comments into code to the body text

I have moved the comments into code to the body text for a better read!
Do not forget about inline highlight the codes with \`, for example:

> You can put a fixed message like "Invalid number" or a message from `ResourceBundle` with `{}`: 
```java
String message() default  "Invalid number";
```

Notice that I've changed a little the text and removed the `{}` from the example, since it is a fixed message