Remove drag message if only one value allowed

Problem/Motivation

This module's widget currently always shows the same description, even if the field only allows one value:
"Drag to re-order taxonomy terms."

This isn't intuitive for single-value fields. It may seem like an edge case to use Tagify for single value fields, but I'm using it for other entity reference fields on the same site, and users are sometimes confused by the numbers appended in a standard Drupal autocomplete field.

Proposed resolution

Add logic to only show this description when the field allows multiple values.