libnm: Override parts of nm-setting-docs.xml

libnm: Override parts of nm-setting-docs.xml

Add "---dbus---" sections to the NMSetting property docs, in the same
style as the plugin docs, parse them out into a file
"nm-setting-docs-overrides.xml", and use them to override the GObject
property docs in nm-setting-docs.xml.

This lets us put more D-Bus-specific information in the setting docs,
without cluttering up the property docs, and it also lets us document
dbus-only properties.