[SPARK-4733] Add missing prameter comments in ShuffleDependency

Add missing Javadoc comments in ShuffleDependency.

Author: Takeshi Yamamuro <linguin.m.s@gmail.com>

Closes #3594 from maropu/DependencyJavadocFix and squashes the following commits:

32129b4 [Takeshi Yamamuro] Fix comments in @aggregator and @mapSideCombine
303c75d [Takeshi Yamamuro] [SPARK-4733] Add missing prameter comments in ShuffleDependency