gdbus: convert doc/manager-api.txt

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

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

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gdbus: convert doc/message-waiting-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>