Consolidate Header Set and Body Set columns in document attribute reference to Header Only

[mojavelinux]

When we're talking about document attributes, all the writer needs to know is whether the document attribute must be set by the end of the header (i.e., Header Only). Otherwise, its implied that the attribute can be set anywhere, assuming it is not locked by the CLI or API. This change will also simplify the visual complexity of these tables.

[mojavelinux]

It's true that there are certain attributes that can be modified even if they are locked. At the moment, this only includes sectnums. This is referred to as a flexible attribute. But that's an entirely separate subject and could be obsolete anyway now that Asciidoctor doesn't number special sections by default. (it now must be explicitly enabled using :sectnums: all)

[mojavelinux]

This issue is intertwined with #1 and I think we should solve both in a single pull request (otherwise, we'll end up with merge hell).

There are several concepts to represent:

• Whether an attribute must be set by the end of the header (i.e., Header Only)
• Whether an attribute is set by default
• Whether an attribute has an implied value if not set (implied by the processor)
• Whether an attribute has an implied value if the value is set to empty
• Allowable values if an attribute only accepts certain values or value type

We also may need to deal with whether an attribute is modifiable separately. I think that's out of scope for this change.

[mojavelinux]

There's also whether an attribute is set when running in embedded mode. There are only a few attributes affected by this, which are: embedded and notitle.

[mojavelinux]

As a follow-up, I think we should pull the implicit document attributes into this page. That puts all the known built-in attributes in a single place. And, besides, some of the attributes on this page are implicit and some on the implicit page are modifiable. So we're creating an artificial division. We will need to address that "modifiable" field though. Perhaps we can bold the attribute name to indicate it cannot be changed. Again, probably a separate issue.

[graphitefriction]

There's also whether an attribute is set when running in embedded mode. There are only a few attributes affected by this, which are: embedded and notitle.

I feel like embedded mode is really overlooked/flying under the radar. Or I just keep missing all the documentation about it 👀 I feel like we need to have a little session where we start with "What is embedded mode", "What uses cases does embedded mode solve", "How do I prepare content that I want to embed", "How do I embed this content", "Where can I embed content", "What are the limitations of embedded mode", What can I configure in embedded mode" .... and so on

I think we should pull the implicit document attributes into this page.

I'm down with with that 👍 It should definitely be a separate issue though!

So we're creating an artificial division.

Agreed

We will need to address that "modifiable" field though. Perhaps we can bold the attribute name to indicate it cannot be changed. Again, probably a separate issue.

Definitely a separate issue