Editor instructions help link points to wrong path

Problem/Motivation

pdfa11y_update_10001() sets the default editor_instructions config value with a help link pointing to /admin/help/pdfa11y. However, the module's help page route is actually /admin/config/media/pdf-accessibility/help.

Any site that has already run the update has a broken link in the editor instructions field. Clicking it results in a 404 (or routes to Drupal core's help page, which is not the same content).

Steps to reproduce

1. Install pdfa11y and run database updates so pdfa11y_update_10001() executes.
2. Go to Administration > Configuration > Media > PDF Accessibility (/admin/config/media/pdf-accessibility).
3. Look at the "Editor instructions" field — the default value contains a link to /admin/help/pdfa11y.
4. Click the link — it returns a 404 or goes to the wrong page.

Proposed resolution

1. Fix the path in pdfa11y_update_10001() so new installs get the correct link (/admin/config/media/pdf-accessibility/help).
2. Add pdfa11y_update_10003() to find and replace the incorrect path in sites that have already run the update. It only modifies the value if it still contains the old path, so sites that have customized the field are unaffected.

Remaining tasks

• Review
• Merge

User interface changes

None. The editor instructions config value is corrected from an incorrect path to the intended path.

API changes

None.

Data model changes

None. A config value is updated in place.