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

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

diff --cc Documentation/asciidoctor-extensions.rb
Simple merge