Fix documentation for `nstack` to more accurately describe what it does now.
Fix documentation for `nstack` to more accurately describe what it does now.

PiperOrigin-RevId: 479409986
Change-Id: Ic7cfadb610e6938bdea58cc01b381fd2bbc21a7c

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