Documentation: move some AsciiDoc parameters into variables

Documentation: move some AsciiDoc parameters into variables

Asciidoctor takes slightly different arguments from AsciiDoc in some
cases.  It has a different name for the HTML backend and the "docbook"
backend produces DocBook 5, not DocBook 4.5.  Also, Asciidoctor does not
accept the -f option.  Move these values into variables so that they can
be overridden by users wishing to use Asciidoctor instead of Asciidoc.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>