[docmaker] Honour empty lines in `<Order>' section element. This greatly improves the readability of the `Synopsis' links. * src/tools/docmaker/content.py (DocBlock::get_markup_words_all): Insert string `/empty/' between items. * src/tools/docmaker/formatter.py (Formatter::section_dump): Make it robust against nonexistent keys. * src/tools/docmaker/tohtml.py (HtmlFormatter::section_enter): Emit empty <td> elements for `/empty/'.