schema: property ReST docs: correctly separate nested list from parent

This fixes the reStructuredText (ReST) output for more complex formats
where we got, for example, a definition list of all properties of a
(section) configuration with one or more properties then having a
sub-format that is also rendered as a list.

Such nested lists are possible in ReST but need to be separated by a
blank line [0].

Without this we got quite a few warnings/errors from the Sphinx/ReST
buildsystem looking like:

> config/datastore/config.rst:19: ERROR: Unexpected indentation.
> config/datastore/config.rst:20: WARNING: Block quote ends without a blank line; unexpected unindent.

[0]: https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#lists-and-quote-like-blocks

Reported-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

proxmox-schema/src/format.rs

@@ -174,6 +174,9 @@ pub fn dump_properties(
                             ParameterDisplayStyle::ConfigSub,
                             &[],
                         );
+                        if !sub_text.is_empty() {
+                            param_descr.push_str("\n\n");
+                        }
                         param_descr.push_str(&sub_text);
                     }
                     Schema::Array(_) => {