[SPARK-17718][DOCS][MLLIB] Make loss function formulation label note clearer in MLlib docs

## What changes were proposed in this pull request?

Move note about labels being +1/-1 in formulation only to be just under the table of formulations.

## How was this patch tested?

Doc build

Author: Sean Owen <sowen@cloudera.com>

Closes #15330 from srowen/SPARK-17718.