[SPARK-14478][ML][MLLIB][DOC] Doc that StandardScaler uses the corrected sample std

## What changes were proposed in this pull request?

Currently, MLlib's StandardScaler scales columns using the corrected standard deviation (sqrt of unbiased variance). This matches what R's scale package does.

This PR documents this fact.

## How was this patch tested?

doc only

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #12519 from jkbradley/scaler-variance-doc.