Improve documentation of <freejoint> element.
Improve documentation of <freejoint> element.

PiperOrigin-RevId: 453192974
Change-Id: I14b02c8c37f6208a67b35efc17887668f6555aef

deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco