Documentation does not specify if variance is population or sample

Hello, just a small suggestion here to specify in the documentation that `Matrix::variance` is calculating the population variance and not the sample variance. Coming from an R background, I was surprised when `Matrix::row_variance` didn't return the same values as R's `var`.

Simply changing the doc comment from "The variance of all the elements of this matrix" to "The *population* variance of all the elements of this matrix" might have helped my confusion.