Adds method documentations back to `MultivariateOnlineSummarizer`, which were present in 1.4 but disappeared somewhere along the way to 1.5.

jkbradley

Author: Feynman Liang <fliang@databricks.com>

Closes #8045 from feynmanliang/SPARK-9755 and squashes the following commits:

af67fde [Feynman Liang] Add MultivariateOnlineSummarizer docs