doc: convert description from attribute to node

When generating manual pages for nmcli the description of different properties is not separated into paragraphs and it does not look nice. That is why i propose these changes.

To have description separated into paragraphs the paragraphs first must be preserved when parsing them from GTK docs in ./tool/generate-docs-nm-settings-docs-gir.py. There the description gets parsed and is put into description subnode (the node above is property - before these changes the description was attribute of the property node rather than separate node - this was done to have a better readable intermediate files).

Along the description subnode description-docbook subnode is also created. It contains the same description as description subnode but the paragraphs are further enclosed between <para></para> tags. This lets the xsltproc tool generate manual pages with separated paragraphs.