### What changes were proposed in this pull request?
Document REFRESH TABLE statement in the SQL Reference Guide.
### Why are the changes needed?
Currently there is no documentation in the SPARK SQL to describe how to use this command, it is to address this issue.
### Does this PR introduce any user-facing change?
Yes.
#### Before:
There is no documentation for this.
#### After:
<img width="826" alt="Screen Shot 2019-09-12 at 11 39 21 AM" src="https://user-images.githubusercontent.com/7550280/64811385-01752600-d552-11e9-876d-91ebb005b851.png">
### How was this patch tested?
Using jykll build --serve
Closes#25549 from kevinyu98/spark-28828-refreshTable.
Authored-by: Kevin Yu <qyu@us.ibm.com>
Signed-off-by: Xiao Li <gatorsmile@gmail.com>
