Changed documentation to clarify Correlation

Fix issue #73, 'What types of correlation does jstat do?'

Clarify Documentation:
- Which standard deviation estimator is calculated by stdev()
- Which variance estimator is calculated by variance()
- Which correlation coefficient is calculated by corrcoeff() (Pearson's
  Rho)

Signed-off-by: Trevor Norris <trev.norris@gmail.com>