add doc comments to providers

from Ross burton

providers/example/geoclue-example.c

 /*
  * Geoclue
- * geoclue-example.c - Example provider
+ * geoclue-example.c - Example provider which doesn't do anything.
  *
  * Author: Iain Holmes <iain@openedhand.com>
  * Copyright 2007 by Garmin Ltd. or its subsidiaries

providers/gpsd/geoclue-gpsd.c

 /*
  * Geoclue
- * geoclue-gpsd.c - Geoclue backend for gpsd
+ * geoclue-gpsd.c - Geoclue Position backend for gpsd
  *
  * Authors: Jussi Kukkonen <jku@o-hand.com>
  * Copyright 2007 by Garmin Ltd. or its subsidiaries

providers/gypsy/geoclue-gypsy.c

 /*
  * Geoclue
- * geoclue-gypsy.c - Geoclue backend for Gypsy
+ * geoclue-gypsy.c - Geoclue backend for Gypsy which provides the Position.
  *
  * Authors: Iain Holmes <iain@openedhand.com>
  * Copyright 2007 by Garmin Ltd. or its subsidiaries

providers/manual/geoclue-manual.c

 /*
  * Geoclue
- * geoclue-manual.c - Manual provider
+ * geoclue-manual.c - Manual address provider
  *
  * Author: Jussi Kukkonen <jku@o-hand.com>
  * Copyright 2008 by Garmin Ltd. or its subsidiaries

providers/yahoo/geoclue-yahoo.c

 /*
  * Geoclue
- * geoclue-yahoo.c - A "local.yahooapis.com"-based Geocode-provider
+ * geoclue-yahoo.c - A "local.yahooapis.com"-based Geocode-provider which
+ * converts from street address to position.
  * 
  * Copyright 2008 by Garmin Ltd. or its subsidiaries
  *