[SPARK-3112][MLLIB] Add documentation and example for StreamingLR

Added a documentation section on StreamingLR to the ``MLlib - Linear Methods``, including a worked example.

mengxr tdas

Author: freeman <the.freeman.lab@gmail.com>

Closes #2047 from freeman-lab/streaming-lr-docs and squashes the following commits:

568d250 [freeman] Tweaks to wording / formatting
05a1139 [freeman] Added documentation and example for StreamingLR

(cherry picked from commit c7252b0097cfacd36f17357d195b12a59e503b35)
Signed-off-by: Xiangrui Meng <meng@databricks.com>