[SPARK-28804][DOCS][SQL] Document DESCRIBE QUERY in SQL Reference

[dilipbiswal]

What changes were proposed in this pull request?

Document DESCRIBE QUERY statement in SQL Reference Guide.

Why are the changes needed?

Currently Spark lacks documentation on the supported SQL constructs causing
confusion among users who sometimes have to look at the code to understand the
usage. This is aimed at addressing this issue.

Does this PR introduce any user-facing change?

Yes.

Before:
There was no documentation for this.

After.

How was this patch tested?

Tested using jykyll build --serve

[SparkQA]

Test build #109473 has finished for PR 25529 at commit 5f1df99.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dilipbiswal]

cc @gatorsmile

[srowen]

Back tick the reserved words

[srowen]

PS you might look at the other similar PRs to add these docs to try to harmonize the style and format a little bit. They're pretty similar. That can mean reviewing others' changes.

[SparkQA]

Test build #109515 has finished for PR 25529 at commit 719c717.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #109925 has finished for PR 25529 at commit 4d6f415.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #109926 has finished for PR 25529 at commit 8fe2b67.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[gatorsmile]

LGTM

Thanks! Merged to master.

[dilipbiswal]

@gatorsmile Thank you for fixing the mistake and merging.