Improve comment clarity in MvNormal._random

Moves the brittle comment in MvRandom._random's docstring to the actual location in the code for clarity.

Author: awray3

pymc3/distributions/timeseries.py

@@ -275,7 +275,6 @@ def _random(self, sigma, mu, size, sample_shape):
         """Implement a Gaussian random walk as a cumulative sum of normals.
         axis = len(size) - 1 denotes the axis along which cumulative sum would be calculated.
         This might need to be corrected in future when issue #4010 is fixed.
-        Lines 291-295 set the starting point of each instance of the random walk to 0."
         """
         if size[len(sample_shape)] == sample_shape:
             axis = len(sample_shape)
@@ -284,6 +283,8 @@ def _random(self, sigma, mu, size, sample_shape):
         rv = stats.norm(mu, sigma)
         data = rv.rvs(size).cumsum(axis=axis)
         data = np.array(data)
+
+        # the following lines center the random walk to start at the origin.
         if len(data.shape) > 1:
             for i in range(data.shape[0]):
                 data[i] = data[i] - data[i][0]