Hi rbatista,
you can use Language overrides to add more text to any existing text or change it as you like.
Here is an article that will help you with Language overrides:
http://kanev.com/blog/22-joomla-language-override
In your case if you look for: "enter your complete details" as value you will see result: BOOKING_FORM_STEP2_TEXT - click on it and it will appear on the left side where the override fields are (Joomla 3).
Change the text: Enter your complete details
to
Enter your complete details<br>SECOND LINE<br>THIRD LINE
This way you can add as much text as you like.
Hope this helps.
Kind regards.