Add useNativeQueryExplain in sql query context documentation

[Mihaylov93]

Fixes #12924.

Description

Added the missing sql query context flag to the documentation.

This PR has:

• been self-reviewed.
• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[abhishekagarwal87]

Thank you for your contribution @Mihaylov93

[clintropolis]

this got merged without running through travis and it appears to be failing spellcheck in other PRs:

    ../docs/querying/sql-query-context.md
       45 | in plan generated by Calcite.|druid.sql.planner.useNativeQueryExplain on the Broker (default: False 

[Mihaylov93]

#12936 There is this PR too

For reference: https://app.travis-ci.com/github/apache/druid/jobs/580521295#L759 While I see previous ones do not have backticks, its a bad position to be in. Missing backticks force us to add these configs to spelling file, which doesn't add much value.

[clintropolis]

I've opened #12940 to fix travis

[abhishekagarwal87]

ah. The fact that PR becomes green, without travis running, is really annoying. thanks for fixing it, @clintropolis.