Update documentation for reward scaling wrappers

[keraJLi]

Description

Changes the documentation of reward scaling wrappers. It mainly removes incorrect or unsubstantiated information.
Affected wrappers are wrappers/stateful_reward.py and wrappers/vector/stateful_reward.py.

Fixes #1272

Type of change

Please delete options that are not relevant.

• Documentation only change (no code changed)

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes

[pseudo-rnd-thoughts]

Thanks for the PR @keraJLi, to clarify what do you mean by their exponential moving average?
To me, this isn't clear what the expected mean is or what exactly the rewards are normalised by?