Missing docstrings

These lines in flatland/env/rail_env.py dont have a doctring to explain their utility https://gitlab.aicrowd.com/flatland/flatland/blob/master/flatland/envs/rail_env.py#L672
https://gitlab.aicrowd.com/flatland/flatland/blob/master/flatland/envs/rail_env.py#L685
https://gitlab.aicrowd.com/flatland/flatland/blob/master/flatland/envs/rail_env.py#L703
https://gitlab.aicrowd.com/flatland/flatland/blob/master/flatland/envs/rail_env.py#L717
https://gitlab.aicrowd.com/flatland/flatland/blob/master/flatland/envs/rail_env.py#L393