message: Update doc to be slightly more specific.
diff: @@ -173,7 +173,7 @@ def einsum(equation, *inputs, **kwargs):
  >>> einsum('ij->ji', m) # output[j,i] = m[i,j]
  # Trace
  - >>> einsum('ii', m) # output[j,i] = trace_i
  + >>> einsum('ii', m) # output[j,i] = trace(m) = sum_i m[i, i]
  # Batch matrix multiplication
  >>> einsum('aij,ajk->aik', s, t) # out[a,i,k] = sum_j s[a,i,j] * t[a, j, k]
  