MuJoCo.Ant-v4.use_contact_forces add doc

[Kallinteris-Andreas]

Description

use_contact_forces was previously not mentioned in the doc, and the Observation Space was misleading because of it
now it does

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [no] I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• [yes] I have commented my code, particularly in hard-to-understand areas
• [yes] I have made corresponding changes to the documentation
• [yes] My changes generate no new warnings
• [Does not apply] I have added tests that prove my fix is effective or that my feature works
• [did not test] New and existing unit tests pass locally with my changes

[pseudo-rnd-thoughts]

Thanks for the PR, that is very helpful