make docs shows warnings in onboarding.rst

[pirapira]

I see warnings in onboarding.rst when I run make docs.

/home/yoichi/src/raiden/docs/onboarding.rst:2: WARNING: Duplicate explicit target name: "state transition".
/home/yoichi/src/raiden/docs/onboarding.rst:2: WARNING: Duplicate explicit target name: "subdispatched".
/home/yoichi/src/raiden/docs/onboarding.rst:2: WARNING: Duplicate explicit target name: "state transition".
/home/yoichi/src/raiden/docs/onboarding.rst:2: WARNING: Duplicate explicit target name: "subdispatched".
/home/yoichi/src/raiden/docs/onboarding.rst:2: WARNING: Duplicate explicit target name: "state transition".
/home/yoichi/src/raiden/docs/onboarding.rst:2: WARNING: Duplicate explicit target name: "subdispatched".
/home/yoichi/src/raiden/docs/onboarding.rst:2: WARNING: Duplicate explicit target name: "state transition".
/home/yoichi/src/raiden/docs/onboarding.rst:2: WARNING: Duplicate explicit target name: "processed".
/home/yoichi/src/raiden/docs/onboarding.rst:2: WARNING: Duplicate explicit target name: "processed".