fix: update mean() docstring and fix einsum performance regression

Author: Abineshabee

sparse/numba_backend/_sparse_array.py

@@ -686,6 +686,7 @@ def mean(self, axis=None, keepdims=False, dtype=None, out=None):
         mean along all axes.
 
         >>> s.mean()
+        <COO: shape=(), dtype=float64, nnz=1, fill_value=0.0>
         """
 
         if axis is None: