Enhancement: Provide Suggestive/Assistive Help for Simple Custom Defined Lists with too many Options

[jimtrue]

Is your feature request related to a problem? Please describe.
When folks use Simple Custom Defined List as a Relationship, it's usually intended for a small manageable list. Adding too many items, really should be a relationship to a Custom Post Type or they should use a Taxonomy.

Describe the solution you'd like
If they start adding more than 10 items to these lists, we should suggest 'Hey, you look like you're creating a lot of items. This would be better served by a relationship to a Custom Post Type or using a Taxonomy.

Ideally, we should also do a 'convert to post-type Relationship' and create the post type with the current items as posts and create the relationship for them.