python: service: explain how to wrap async methods
python: service: explain how to wrap async methods

Change-Id: Ib78bcf3dc4f3b79aedfa9afd75d584a465e4ea23
Reviewed-on: http://gerrit.aldebaran.lan/37911
Reviewed-by: cgestes <ddb78fe00abb42f91254c16a0860e38eb0781789@aldebaran-robotics.com>
Tested-by: cgestes <ddb78fe00abb42f91254c16a0860e38eb0781789@aldebaran-robotics.com>

bsautron/libqi,aldebaran/libqi,vbarbaresi/libqi,aldebaran/libqi,aldebaran/libqi