Improve HDHomePluginItem documentation.
Improve HDHomePluginItem documentation.

Fixes: NB#116713 - HdHomePluginItem::show-settings signal needs real
documentation
Fixes: NB#122223 - libhildondesktop's
hd_home_plugin_item_heartbeat_signal_add() needs more documentation
Fixes: NB#122429 - libhildondesktop docu should say whether widgets may
use Clutter

Cordia/libhildondesktop,android-808/libhildondesktop,Cordia/libhildondesktop,android-808/libhildondesktop,android-808/libhildondesktop