Rename C sparse structure to M in mjdata, improve docstrings.

PiperOrigin-RevId: 758636638
Change-Id: If78acc423601d2911f514929b27f7b6d0af9ef58

src/engine/engine_island.c

@@ -537,7 +537,7 @@ void mj_island(const mjModel* m, mjData* d) {
 
   // inertia: block-diagonalize both iLD <- qLD and iM <- qM
   mju_blockDiagSparse(d->iLD, d->iM_rownnz, d->iM_rowadr, d->iM_colind,
-                      d->qLD,  d->C_rownnz, d->C_rowadr, d->C_colind,
+                      d->qLD,  d->M_rownnz, d->M_rowadr, d->M_colind,
                       nidof, nisland,
                       d->map_idof2dof, d->map_dof2idof,
                       d->island_idofadr, d->island_idofadr,