Change description from textfield to text area and remove limit

Currently description is limited to 512 characters. If webforms is being used to respesent an online application format the help text required for a given field often breaks this limit.

Given the increase in limit it makes sense for the intormation to be provided in a textarea. Once we have larger fields the content contained with in often needs to be formated. This introduces a new function which wraps the existing filter function and then converts markup using the check markup function.