This PR includes:
* Update SparkR:::glm, SparkR:::summary API docs.
* Update SparkR machine learning user guide and example codes to show:
  * supporting feature interaction in R formula.
  * summary for gaussian GLM model.
  * coefficients for binomial GLM model.

mengxr

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #9727 from yanboliang/spark-11684.