Add help text to min and max order quantity fields

Problem/Motivation

With the addition of ✨ Entity traits don't show descriptions in their entity form fields. Active in Commerce Core, it is now possible to provide help text on fields added by traits.
We should show help text to indicate to the site content editors that if the min and max fields are left blank, then the order quantity will not be limited.

Steps to reproduce

Proposed resolution

Add a call to BundleFieldDefinition::setDescription() in the ::buildFieldDefinitions() method of the respective traits.
e.g.

      // .......
      $fields['maximum_order_quantity'] = BundleFieldDefinition::create('integer')
        ->setLabel(t('Maximum quantity per order'))
        ->setDescription(t('Leave blank for unlimited maximum quantity.'))
      // ......

Remaining tasks

User interface changes

API changes

Data model changes