[SPARK-20881][SQL] Clearly document the mechanism to choose between two sources of statistics

## What changes were proposed in this pull request?

Now, we have two sources of statistics, i.e. Spark's stats and Hive's stats. Spark's stats is generated by running "analyze" command in Spark. Once it's available, we respect this stats over Hive's.

This pr is to clearly document in related code the mechanism to choose between these two sources of stats.

## How was this patch tested?

Not related.

Author: Zhenhua Wang <wzh_zju@163.com>

Closes #18105 from wzhfy/cboSwitchStats.