Add docstring to MujocoEnv.__init__()

[Kallinteris-Andreas]

Description

adds a docstring to the init of MujocoEnv, there has been plenty of confusion on what some arguments do such as frame_skip

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• Documentation change

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes