doc/device-api: Add GattServices
doc/device-api: Add GattServices

This patch introduces the GattServices property that contains the object
paths of exported GattService1 objects. The purpose of this property is
to signal when all GATT services on a remote device have been
discovered.

mapfau/bluez,pkarasev3/bluez,silent-snowman/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,silent-snowman/bluez,ComputeCycles/bluez,ComputeCycles/bluez,mapfau/bluez,pstglia/external-bluetooth-bluez,mapfau/bluez,silent-snowman/bluez,ComputeCycles/bluez,ComputeCycles/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,mapfau/bluez,pkarasev3/bluez