docs: clarify example EPUB’s licence

[robinwhittleton]

SE productions are in the US public domain, but are not CC0. The parts of the production that are explicitly CC0 are the parts the producer has created for the work: typically the metadata.

You can check the licence for your example book at https://github.com/standardebooks/sarah-louisa-forten-purvis_poetry/blob/master/LICENSE.md.

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

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[robinwhittleton]

See also #3706.