Explain vocabulary types options in creation popup

Currently a user can choose between simple child and hierarchical vocabularies, but the difference between them is not explained anywhere.

=> Replace drop-down list by a radio selector with the following options:

• Simple vocabulary: one-level only vocabulary.
• Child vocabulary: used to add another level to an existing simple or child vocabulary.
• Hierarchical vocabulary: multi-level vocabulary. Recommended for multi-level lists over a simple and child vocabularies combination.