gdbus: convert doc/handsfree-audio-api.txt

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gdbus: convert doc/location-reporting-api.txt

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gdbus: convert doc/modem-api.txt

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gdbus: convert doc/voicecallmanager-api.txt

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>