Added Docstrings to the BabyAI envs

[BolunDai0216]

Description

Added docstrings to the BabyAI environments.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

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

[pseudo-rnd-thoughts]

Damn this is very impressive, I only have a question on the environment names

[pseudo-rnd-thoughts]

To fix the build issues, check that the isinstance(spec.entry_point, str) before the variable