docs(config): clarify overwrite.cli.url description

[joshtrichards]

Summary

Clarify the overwrite.cli.url description in config/config.sample.php so it better reflects how Nextcloud uses the value during CLI and background execution.

The updated text:

• explains that the setting is used when generating URLs outside a normal web request
• calls out common contexts like background jobs, occ, and notifications
• notes that installation seeds the value from the installer’s current access context when it is not already preseeded
• warns that the inferred value may not match the public URL users actually use and thus may need manual adjustment

Notes

This is a documentation-only change. No runtime behavior is changed.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI