How to overwrite a field’s description

Hello

The description text for a field only allows 200 characters. I need to add a long text to assist the users understand the purpose of the field.

If I do a view source of the page I can that there is a <TD> with id= hi_applicationid_c that holds the descriptions as its title.

I was hoping that I using the getElementById can change the title but the element is null.

var element = document.getElementById("hi_applicationid_c");

Any suggestions on how to extend the field description?

Thank you.