[SPARK-31612][SQL][DOCS] SQL Reference clean up

[huaxingao]

What changes were proposed in this pull request?

SQL Reference cleanup

Why are the changes needed?

To complete SQL Reference

Does this PR introduce any user-facing change?

Fix the following stuff:

• Example -> Examples
• SET DBPROPERTIES ( property_name = property_value, ... ) -> SET DBPROPERTIES ( property_name = property_value [ , ... ] )
  // be consistent with format. brackets [ and ] indicate optional parts, and dots ... mean that the
  // preceding element can be repeated.
• col_spec [ , col_spec ... ] -> col_spec [ , ... ] // same as above, be consistent with format.
• [USING data_source] -> [ USING data_source ] // be consistent with format. space between [ and text
• change function_name format to make it consistent with others
• from -> FROM // uppercase keyword
• { { VALUES ( { value | NULL } [ , ... ] ) [ , ( ... ) ] } | query } -> { VALUES ( { value | NULL } [ , ... ] ) [ , ( ... ) ] | query } // remove unnecessary { }
• DROP ( DATABASE | SCHEMA ) -> DROP { DATABASE | SCHEMA }
  // Braces { and } and vertical lines | indicate that you must choose one alternative
• fix a few link and typo too.

Also updated sql-ref-syntax-qry.html

before

after

How was this patch tested?

Manually build and check

[huaxingao]

cc @gatorsmile @maropu
In the first round of clean-up, I will focus on syntax and typos. After I finish this, I will remove all the html syntax following this PR #28414. Then change HTML tables to MD tables following this PR #28415
I have added a couple of <code> </code> in this PR, because when I did this, I didn't know we will remove html syntax. I will just leave these <code> </code>, will remove all the html syntax in the second round of clean-up.

[SparkQA]

Test build #122109 has finished for PR 28417 at commit 92bda1b.

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

[SparkQA]

Test build #122114 has finished for PR 28417 at commit 5b8b221.

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

[maropu]

Thanks! Merged to master/3.0.

[huaxingao]

Thanks! @maropu @srowen

[maropu]

Ur, plz add what's cleaned up in the PR description (& the jira).

[huaxingao]

Updated.