docs: unify "nm-property-infos-*.xml" and "nm-settings-docs-*.xml" (root element)

There is no need that two XML files that essentially hold similar
information are fundamentally different. Make them more alike.
This way, we can use the same tools that operate on either of
these input files.

libnm/generate-docs-nm-settings-docs-merge.py

@@ -107,7 +107,7 @@ output_xml_file = sys.argv[1]
 
 xml_roots = list([ET.parse(f).getroot() for f in sys.argv[2:]])
 
-assert(all([root.tag.startswith('nm-') for root in xml_roots]))
+assert(all([root.tag == 'nm-setting-docs' for root in xml_roots]))
 
 settings_roots = list([node_to_dict(root, 'setting', 'name') for root in xml_roots])