[SPARK-17080][SQL][FOLLOWUP] Improve documentation, change buildJoin method...

[SPARK-17080][SQL][FOLLOWUP] Improve documentation, change buildJoin method structure and add a debug log

## What changes were proposed in this pull request?

1. Improve documentation for class `Cost` and `JoinReorderDP` and method `buildJoin()`.
2. Change code structure of `buildJoin()` to make the logic clearer.
3. Add a debug-level log to record information for join reordering, including time cost, the number of items and the number of plans in memo.

## How was this patch tested?

Not related.

Author: wangzhenhua <wangzhenhua@huawei.com>

Closes #17353 from wzhfy/reorderFollow.