asciidoctor: fix user-manual to be built by `asciidoctor`

asciidoctor: fix user-manual to be built by `asciidoctor`

The `user-manual.txt` is designed as a `book` but the `Makefile` wants
to build it as an `article`. This seems to be a problem when building
the documentation with `asciidoctor`. Furthermore the parts *Git
Glossary* and *Appendix B* had no subsections which is not allowed when
building with `asciidoctor`. So lets add a *dummy* section.

Signed-off-by: 마누엘 <nalla@hamal.uberspace.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>