Improve documentation of MotionCheck Behaviour.

[chenkins]

Target

• Improve documentation
• If possible, foster ideas to improve performance (faster implementation or other solutions)

Source code

• RailEnv documentation says

The actions of the agents are executed in order of their handle to prevent deadlocks and to allow them to learn relative priorities.
This is not true any more, see https://github.com/flatland-association/flatland-rl/blob/main/notebooks/Agent-Close-Following.ipynb

Book / notebooks/unit tests

• behaviour should be documented in the core documentation (or at least be consistent with the current implementation)
• behaviour should be better documented by unit tests