One root the document itself declares (ADR-0001 decision 11).
A document-declared root is a top-level datamodel entry: an id the
document's own guards and assigns read, an optional expr the root
starts with, and an optional description for a human reading the
document rather than for the chart - it is prose, never compiled and
never emitted (StatifierBlocks.Compiler.DeclaredRoots.document_declarations/1
drops it). This is the document's own declaration surface, distinct from
the compile call's :declare option, which leads it
(StatifierBlocks.Compiler's moduledoc).