rng2asciidocman.xsl: more compact notation for defaults

Inspired by fence-virt project [1,2].

[1] http://fence-virt.git.sourceforge.net/git/gitweb.cgi?p=fence-virt/fence-virt;a=blob;f=client/options.c;h=cd975ae882bda389acb8ba293070ba1f28e9df7c;hb=HEAD#l451
[2] https://github.com/ryan-mccabe/fence-virt/blob/master/client/options.c#L451

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>