Merge branch 'ma/user-manual-markup-update'

The markup used in user-manual has been updated to work better with asciidoctor. * ma/user-manual-markup-update: user-manual.txt: render ASCII art correctly under Asciidoctor asciidoctor-extensions.rb: handle "book" doctype in linkgit user-manual.txt: change header notation user-manual.txt: add missing section label