Doc: asciidoc.reference: extend, use more abstract styles

• italics -> emphasis (in terms of DocBook [1], splitting markup for files into a high-level markup section)
• bold -> strong (ditto)
• add some more high-level markups based on Ken's feedback

[1] http://docbook.org/tdg/en/html/emphasis.html

Contributed-by: Ken Gaillot <kgaillot@redhat.com>
Signed-off-by: Jan Pokorný <jpokorny@redhat.com>