Duplexing support is incomplete in PrintTicket and PrintCapabilities

I initially logged this as a Documentation issue in https://github.com/dotnet/dotnet-api-docs/issues/3282#issuecomment-540858696

But that documentation issue has since been Closed with the recommendation that this be logged as a Feature Request instead. So here it is:

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The documentation source files dotnet-api-docs/xml/System.Printing/PrintTicket.xml and PrintCapabilities.xml both describe printer Duplexing support, and both have a Remark saying:

This property corresponds to the Print Schema's `JobDuplexAllDocumentsContiguously` keyword, not the `DocumentDuplex` keyword.

I'm puzzled that the printing schema contains _two_ keywords for duplex support but the accompanying classes only support _one_ of them. Ideally I'd like the classes to be modified to support _both_ schema keywords, or (failing that) some new note in the documentation suggesting what to do if the Printer Driver reports duplex behaviour using `DocumentDuplex` rather than `JobDuplexAllDocumentsContiguously`. Both classes are sealed, so cannot be extended to add the new behaviour - the classes themselves would need to change.