P1673: vector_norm2: should init be inside or outside the sqrt?

[fnrizzi]

In the impl we have:

  // Compute the sum of squares using an algorithm that avoids
  // underflow and overflow by scaling.
  auto ssq_res = vector_sum_of_squares(exec, x, ssq_init);
  using std::sqrt;
  return init + ssq_res.scaling_factor * sqrt(ssq_res.scaled_sum_of_squares);
}

but in the spec we have:

For N equal to v.extent(0), the mathematical expression for the algorithm is sqrt(init + s), 

[mhoemmen]

In P1673R7 (to be submitted tonight), I added wording to the spec to clarify init.