Clarify server requirements for .htaccess file editing

[JSCHLTE]

Context

This PR updates the description for the SEO tools editor to better clarify when the .htaccess file can be edited. The original wording implied that just having the file is enough, but the editor is actually only available on certain server types (like Apache).

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Summary

Changelog label: changelog: other

This PR can be summarized in the following changelog entry:

• Clarifies that the .htaccess file can only be edited if the server supports it, such as Apache, not just based on the file's presence.

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Relevant technical choices:

• Updated the desc string in admin/pages/tools.php to clarify that server compatibility (not just file existence) determines access to the editor.

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Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

1. Go to the SEO > Tools screen in the admin dashboard.
2. Look for the SEO File Editor tool.
3. Confirm the new description reads:

   This tool lets you edit important SEO-related files like robots.txt. Editing .htaccess is only available on servers that support it (e.g., Apache).

4. Confirm the .htaccess editor still only appears when using Apache.
5. Confirm there are no PHP or UI errors.

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Relevant test scenarios

• Changes should be tested with the browser console open
• Not dependent on post types or custom editors
• Does not vary by browser
• Not applicable to multisite

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Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

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Impact check

This PR affects the Tools screen in the WordPress admin area. It does not change logic, only visible text, so regression risk is low.

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UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

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Other environments

• This PR does not affect Shopify

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Documentation

• No new documentation needed — this is a UI clarification only

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Quality assurance

• I have tested this code to the best of my abilities.
• I have verified the text renders correctly.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.

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Innovation

• No innovation project is applicable for this PR.

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Fixes #12065