Add Contextual Help for 'Available Countries' Setting in Address Module

Problem/Motivation

Users have reported in slack confusion arising from the Address module’s behavior wherein the 'country' field does not display when only one default country is configured. This lack of visibility can lead users to mistakenly think that the module lacks the ability to select a country, potentially deterring them from utilizing the module under the impression that it does not meet their needs.

Steps to reproduce

1. Configure an address field with only one default country in a content type.
2. Observe the absence of the 'country' field in the address form on the user interface.

Proposed resolution

To improve clarity, a field description or help text should be introduced next to the 'default country' configuration in the Address module. This text should explain the behavior that occurs when only one country is set as the default and provide a link to the relevant documentation for further guidance.

Remaining tasks

Create or update documentation to describe the behavior of the Address module, specifically detailing the conditions under which the 'country' field is hidden when a single default country is selected. This should be made clear both in the module's UI and the official documentation.

User interface changes

Implementation of a field description or help text next to the 'default country' setting in the Address module configuration, with a link to relevant documentation for user assistance.

API changes

None anticipated as this is a UI and documentation update.

Data model changes

None required for this update.

This issue is in the spirit of Drupal core improvements such as the "Add link to Update module documentation about installer settings" ✨ Add link to Update module documentation about installer settings Needs work issue, aiming to enhance user understanding and usability of the Drupal interface.