[docstring] fix UDD docstrings

[marcvivesmassana]

Note

Improves documentation for UserDefinedDynamic in user_defined_dynamics.py.

• Clarifies input_vars description and examples; removes outdated cross-reference
• Rewrites output_vars description, explicitly enumerating outputs and adding actuatorDisk_<DISK_ENTITY_NAME>_thrustMultiplier and actuatorDisk_<DISK_ENTITY_NAME>_torqueMultiplier; retains caution about solver-side effects

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