mju_boxQP only reads from the lower triangle of the input Hessian.

- Before this change, an asymmetric Hessian would lead to solver failure. After this change the Hessian is symmetric by definition (upper triangle is ignored).
- API documentation was updated to reflect this contract.

PiperOrigin-RevId: 477135411
Change-Id: I60d011d36853afebac26c359771c217baf101360

include/mujoco/mujoco.h

@@ -1032,6 +1032,7 @@ MJAPI int mju_eig3(mjtNum eigval[3], mjtNum eigvec[9], mjtNum quat[4], const mjt
 //     the initial value of res is used to warmstart the solver
 //     R must have allocatd size n*(n+7), but only nfree*nfree values are used in output
 //     index (if given) must have allocated size n, but only nfree values are used in output
+//     only the lower triangles of H and R and are read from and written to, respectively
 //     the convenience function mju_boxQPmalloc allocates the required data structures
 MJAPI int mju_boxQP(mjtNum* res, mjtNum* R, int* index, const mjtNum* H, const mjtNum* g, int n,
                     const mjtNum* lower, const mjtNum* upper);