docs: rework generating property infos to use <description> element

The "generate-docs-nm-property-infos.py" script parses the tags like
"---nmcli---" and generates an XML.

Rework it:

- don't put long text descriptions in a "description=" XML attribute.
  Instead, use an XML element. That is in line with what
  "generate-docs-nm-settings-docs-gir.py" does, which generates
  a similar file.

- if there is no <description-docbook> element, generate one based
  on <description>. That is important, because we want to create
  paragraphs.
  It's also important because "generate-docs-nm-settings-docs-gir.py"
  tends to generate <description-docbook> from the libnm/gir data.
  However, if you specify a "---nmcli---" override, then that should
  automatically apply to <description> and <description-docbook>.

tools/generate-docs-nm-property-infos.py

@@ -116,7 +116,7 @@ def get_file_infos(source_files):
 
 
 KEYWORD_XML_TYPE_NESTED = "nested"
-KEYWORD_XML_TYPE_NODE = "node"
+KEYWORD_XML_TYPE_ELEM = "elem"
 KEYWORD_XML_TYPE_ATTR = "attr"
 
 keywords = collections.OrderedDict(
@@ -127,12 +127,13 @@ keywords = collections.OrderedDict(
         ("values", KEYWORD_XML_TYPE_ATTR),
         ("default", KEYWORD_XML_TYPE_ATTR),
         ("example", KEYWORD_XML_TYPE_ATTR),
-        ("description", KEYWORD_XML_TYPE_ATTR),
+        ("description", KEYWORD_XML_TYPE_ELEM),
         ("description-docbook", KEYWORD_XML_TYPE_NESTED),
     ]
 )
 
 
+
 def keywords_allowed(tag, keyword):
     # certain keywords might not be valid for some tags.
     # Currently, all of them are always valid.
@@ -163,7 +164,7 @@ def write_data(tag, setting_node, line_no, parsed_data):
             # Set as XML nodes. The input data is XML itself.
             des = ET.fromstring("<%s>%s</%s>" % (k, v, k))
             property_node.append(des)
-        elif xmltype == KEYWORD_XML_TYPE_NODE:
+        elif xmltype == KEYWORD_XML_TYPE_ELEM:
             node = ET.SubElement(property_node, k)
             node.text = v
         elif xmltype == KEYWORD_XML_TYPE_ATTR:
@@ -171,6 +172,17 @@ def write_data(tag, setting_node, line_no, parsed_data):
         else:
             assert False
 
+    if (
+        parsed_data.get("description", None) is not None
+        and parsed_data.get("description-docbook", None) is None
+    ):
+        # we have a description, but no docbook. Generate one.
+        node = ET.SubElement(property_node, 'description-docbook')
+        for l in re.split("\n", parsed_data["description"]):
+            paragraph = ET.SubElement(node, "para")
+            paragraph.text = l
+
+
 
 kwd_first_line_re = re.compile(r"^ *\* ([-a-z0-9]+): (.*)$")
 kwd_more_line_re = re.compile(r"^ *\*( *)(.*?)\s*$")