Documentation: rename docbook-xsl-172 attribute to git-asciidoc-no-roff

Documentation: rename docbook-xsl-172 attribute to git-asciidoc-no-roff

It seems that the ability to use raw roff codes in asciidoc.conf
was eliminated by docbook-xsl 1.72.0 _and later_. Unlike the
1.72.0-specific XSLT problem, this behavior was not reverted in
later releases.

This patch aims to make it clear that the affected asciidoc
attribute (flag) can be reasonably used with docbook-xsl versions
other than 1.72.0.

Also, document which make variables should be set for various
versions of asciidoc and docbook-xsl.

Testing done with asciidoc 8.3.1 and docbook-xsl 1.74.0.

Signed-off-by: Chris Johnsen <chris_johnsen@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>