gh-156388: Docs: clarify DocumentType identifiers - #157646

Thanks for confirming this point in the issue discussion. I’ve submitted #157646 to document that mapping explicitly.

For parsed XML documents, the combinations correspond as follows:

• publicId and systemId non-None → PUBLIC
• publicId is None, systemId non-None → SYSTEM
• both are None → no external identifier
• publicId non-None, systemId None → not produced by parsing a valid XML DOCTYPE

I also noted in the PR that createDocumentType() does not validate the identifiers, so programmatically created DocumentType objects may have other combinations.

I’ll leave the PR open for maintainer review and follow any requested changes.