Add text that "Don't allow self-enrollment" setting is not available for paid courses

[renatho]

Proposed Changes

• As suggested in https://github.com/Automattic/sensei-pro/pull/2443#issuecomment-1787716750, it adds a text explaining that the setting is not available for paid courses.

Testing Instructions

1. Navigate to the course editor.
2. Open the settings.
3. Make sure the text is updated.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[github-actions]

WordPress Dependencies Report

The github-action-wordpress-dependencies-report action has detected some script changes between the commit 348fb2b and trunk. Please review and confirm the following are correct before merging.

Script Handle	Added Dependencies	Removed Dependencies	Total Size	Size Diff
js/admin/course-edit.js			10.2 kB	+1 B ( +0.01% 🔼 )

This comment was automatically generated by the github-action-wordpress-dependencies-report action.

[codecov]

Codecov Report

Merging #7250 (348fb2b) into trunk (b58d51a) will not change coverage.
Report is 1 commits behind head on trunk.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              trunk    #7250   +/-   ##
=========================================
  Coverage     50.46%   50.46%           
  Complexity    10923    10923           
=========================================
  Files           605      605           
  Lines         46027    46027           
  Branches        402      402           
=========================================
  Hits          23228    23228           
  Misses        22472    22472           
  Partials        327      327           

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[merkushin]

Looks as expected :)