doc: change the markup of paragraphs following a nested list item

doc: change the markup of paragraphs following a nested list item

Asciidoctor and asciidoc.py have different behaviors when a paragraph
follows a nested list item. Asciidoctor has a bug[1] that makes it keep a
plus sign (+) used to attached paragraphs at the beginning of the paragraph.

This commit uses workarounds to avoid this problem by using second level
definition lists and open blocks.

[1]:https://github.com/asciidoctor/asciidoctor/issues/4704

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>