[examples] correct comments on VB parameters

o most important to manually set is "W0"
o "W0" is the inverse of the prioe covariance; it should be set
  such that "inv(W0)" is much smaller than the expected covariances.
  Otherwise, the resulting covariances are dominated by "W0" which is
  unwanted in the examples presented.
o "m" has a better default initialization now and is therefore not too
  critical any more.