[rllib] Better document APIs to access policy state (#2932)
[rllib] Better document APIs to access policy state (#2932)

* fix

* doc

* example

* up

pcmoritz/ray-1,richardliaw/ray,atumanov/ray,pcmoritz/ray-1,richardliaw/ray,atumanov/ray,ujvl/ray-ng,ray-project/ray,robertnishihara/ray,ray-project/ray,robertnishihara/ray,robertnishihara/ray,ray-project/ray,stephanie-wang/ray,richardliaw/ray,pcmoritz/ray-1,pcmoritz/ray-1,stephanie-wang/ray,stephanie-wang/ray,stephanie-wang/ray,pcmoritz/ray-1,ray-project/ray,stephanie-wang/ray,atumanov/ray,ujvl/ray-ng,stephanie-wang/ray,robertnishihara/ray,robertnishihara/ray,richardliaw/ray,stephanie-wang/ray,richardliaw/ray,robertnishihara/ray,atumanov/ray,pcmoritz/ray-1,ujvl/ray-ng,ray-project/ray,ray-project/ray,atumanov/ray,stephanie-wang/ray,robertnishihara/ray,ray-project/ray,pcmoritz/ray-1,atumanov/ray,atumanov/ray,richardliaw/ray,atumanov/ray,robertnishihara/ray,stephanie-wang/ray,robertnishihara/ray,ujvl/ray-ng,ray-project/ray,pcmoritz/ray-1,richardliaw/ray,richardliaw/ray