[SPARK-18705][ML][DOC] Update user guide to reflect one pass solver for L1 and elastic-net

## What changes were proposed in this pull request?

WeightedLeastSquares now supports L1 and elastic net penalties and has an additional solver option: QuasiNewton. The docs are updated to reflect this change.

## How was this patch tested?

Docs only. Generated documentation to make sure Latex looks ok.

Author: sethah <seth.hendrickson16@gmail.com>

Closes #16139 from sethah/SPARK-18705.