Constant CLASS_DOCS_ONLY_MARKER 

pub const CLASS_DOCS_ONLY_MARKER: &str = ".__MX_CLASS_DOCS_ONLY__";

Opens a class’s roxygen block, in ClassBlockEntry::head, that only carries the impl docs: the block of an S3 or vctrs class without a constructor that the package names and builds itself (class = "..."). The writer writes the block, up to its closing NULL, only when the type’s class_docs or class_doc_lines say something beyond the page and export tags, so a readers-only impl adds no page (#1918). Spelled the same as miniextendr_macros’s CLASS_DOCS_ONLY_MARKER.