[SPARK-16320][DOC] Document G1 heap region's effect on spark 2.0 vs 1.6
[SPARK-16320][DOC] Document G1 heap region's effect on spark 2.0 vs 1.6

## What changes were proposed in this pull request?

Collect GC discussion in one section, and documenting findings about G1 GC heap region size.

## How was this patch tested?

Jekyll doc build

Author: Sean Owen <sowen@cloudera.com>

Closes #14732 from srowen/SPARK-16320.

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