Clarify that full linkage graph must be rooted in primary data

[jelhan]

The current wording in specification does not enforce that a graph of related resources in a Compound Document is rooted in primary data. The requirement could be also interpreted as if two included resources referencing each other fulfill it. This has been very well explained by @beauby in #1035.

The original pull request created by @beauby more than four years ago got stale. But the issue is still the same in current specification.

I updated the @beauby's proposal given in #1035 to target v1.1. I also tried to slightly improve the wording.

Closes #1035

[gabesullice]

Thanks for reviving this issue @beauby. What do you think about removing the note altogether and simplifying the paragraph to:

Every included resource object MUST be identified via a chain of
relationships originating in a document's primary data. This means that
compound documents require "full linkage" and that no resource object can be
included without a direct or indirect relationship to the document's primary
data.

[jelhan]

I like it. It seems to be more precise than the current version while still being easier to understand. I think you managed to achieve both at the same time. 👏

[dgeb]

I agree that @gabesullice's version is a nice improvement. 💯

@jelhan please feel free to update this PR with Gabe's wording or, if you prefer, close it so that he can create a separate PR.

[jelhan]

@jelhan please feel free to update this PR with Gabe's wording

Update the PR. Let me know if I should stash the commits or if you do it anyways as part of merging the pull request.

[dgeb]

Update the PR. Let me know if I should stash the commits or if you do it anyways as part of merging the pull request.

I just merged + squashed to keep it simple.

Thanks @jelhan and @gabesullice!