[SPARK-2843][MLLIB] add a section about regularization parameter in ALS

atalwalkar srowen

Author: Xiangrui Meng <meng@databricks.com>

Closes #2064 from mengxr/als-doc and squashes the following commits:

b2e20ab [Xiangrui Meng] introduced -> discussed
98abdd7 [Xiangrui Meng] add reference
339bd08 [Xiangrui Meng] add a section about regularization parameter in ALS

(cherry picked from commit e0f946265b9ea5bc48849cf7794c2c03d5e29fba)
Signed-off-by: Xiangrui Meng <meng@databricks.com>