[SPARK-31502][SQL][DOCS] Document identifier in SQL Reference

### What changes were proposed in this pull request?
Document identifier in SQL Reference

### Why are the changes needed?
make SQL Reference complete

### Does this PR introduce any user-facing change?
Yes
<img width="1049" alt="Screen Shot 2020-04-23 at 11 14 10 PM" src="https://user-images.githubusercontent.com/13592258/80180695-2f2a4f00-85b8-11ea-819b-f96872956d05.png">

<img width="1050" alt="Screen Shot 2020-04-23 at 11 32 32 PM" src="https://user-images.githubusercontent.com/13592258/80182062-e6c06080-85ba-11ea-9502-1c38358c97c9.png">

### How was this patch tested?
Manually build and check

Closes #28277 from huaxingao/identifier.

Authored-by: Huaxin Gao <huaxing@us.ibm.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>