AUTO_SECTION_NODE and sectioning commands?

[Aidan Zhai]

1. AUTO_SECTION_NODE says:

                “If set, insert nodes for sectioning commands lacking
a corresponding node.”

2. @heading is documented in the "Chapter Structuring" chapter, so I
expected it to be treated as a sectioning command.

However, I found that `AUTO_SECTION_NODE` doesn't generate a node for
`@heading`, which confuses me.