Improve mj_addM and associated test

- Updated docs to clarify that only the lower triangle is touched
- The function was unnecessarily adding the upper triangle in the sparse case
- The test used an unnecessarily large model and stepping it leading to timeouts under ASAN

PiperOrigin-RevId: 798528736
Change-Id: I592cc17dfb62379bb0cd6b332a8eb7e8b0ef6355

python/mujoco/introspect/functions.py

@@ -3001,7 +3001,7 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
                  nullable=True,
              ),
          ),
-         doc='Add inertia matrix to destination matrix. Destination can be sparse or dense when all int* are NULL.',  # pylint: disable=line-too-long
+         doc='Add inertia matrix to destination matrix (lower triangle only). Destination can be sparse or dense when all int* are NULL.',  # pylint: disable=line-too-long
      )),
     ('mj_applyFT',
      FunctionDecl(