How to add placeholder to all Drupal 7 text forms in node page?
I take it by wanting a Placeholder that the text is going to be removed by javaScript when the use enters text ?

If so, there are a couple of modules for that, for example :
https://www.drupal.org/project/field_placeholder

This will give you an extra setting in the field setting form so you wont have to use your description, allowing it to be used for the description.