Allow default text to be a multi-line text box

[DamianSuess]

As a user, I would like for the "default text" box to be multi-line, just like the actual textbox used to input our Descriptions, Acceptance Criteria, etc.

Perhaps, VSTS does not allow this to be modified?

Reproduce

1. Admin Settings > Process (select process) > Product Backlog Item
2. Acceptance Criteria > Markdown (control extension) > Edit Field
3. Options > Default Value

This field is a single line, whereas the actual box is a multi-line textbox.

Example usage:

For "Acceptance Criteria" we've used the following default text

## Requirements
 * Definitions of done here

## Out of Scope
 * Scope-creep examples here