[SPARK-9582] [ML] LDA cleanups

Small cleanups to recent LDA additions and docs.

CC: feynmanliang

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

Closes #7916 from jkbradley/lda-cleanups and squashes the following commits:

f7021d9 [Joseph K. Bradley] broadcasting large matrices for LDA in local model and online learning
97947aa [Joseph K. Bradley] a few more cleanups
5b03f88 [Joseph K. Bradley] reverted split of lda log likelihood
c566915 [Joseph K. Bradley] small edit to make review easier
63f6c7d [Joseph K. Bradley] clarified log likelihood for lda models