docbook: render 4th section level for manpages

In the manpage output the sections get rendered as refsections, but
those only go up to level 3:

> In DocBook, RefSect3 is the lowest-level section allowed in a
> RefEntry. There is no element analogous to a Sect4.
-- https://tdg.docbook.org/tdg/4.5/refsect3

While nesting refsect3 would work output wise, the DTD schema does not
allows this, and disabling schema validation can lead to graver
issues, so rather avoid that and use a formalpara [0], a paragraph
with a title, instead, but only for the title, as the inner content
will be a simpara (simple paragraph) that cannot be nested inside a
formalpara, but the resulting rendering works out well enough.

[0]: https://tdg.docbook.org/tdg/4.5/formalpara

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

asciidoc/pve-docbook.conf

@@ -5,6 +5,7 @@
 #
 # modified for Proxmox VE docs
 # - add thumbnail attribute
+# - render sect4 sections in manpages as formalpara title
 
 [miscellaneous]
 outfilesuffix=.xml
@@ -693,6 +694,16 @@ template::[docinfo]
 |
 </refsect3>
 
+# Proxmox: add custom section 4 level for manpages, as there is no refsect4 and
+# the XML DTD schema does not allows nesting refsect3 (even if it would render
+# OK). Use a formalpara, which is a paragraph with a title. As docbook always
+# outputs a simpara (simple paragraph, very limited) for the inner content, we
+# cannot nest that either, but rather just create an empty formalpara which can
+# then be followed by whatever the original inner content is.
+[sect4]
+<formalpara{id? id="{id}"}{role? role="{role}"}><title>{title}</title><para></para></formalpara>
+|
+
 endif::doctype-manpage[]
 
 #-------------------------