[Docs] Clarify uisng an indexSpec for MSQ queries

[techdocsmith]

Fixes #XXXX.

Description

Clarify reference to MSQ/Druid SQL around indexSpec in the query context.

The reference to indexSpec within a query context for SQL was ambiguous. Made an attempt to clarify that you would create a query context object called indexSpec to set the values for Druid SQL ingestion

Release note

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Key changed/added classes in this PR

• MyFoo
• OurBar
• TheirBaz

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This PR has:

• [ ] been self-reviewed.
  • [ ] using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• [ ] added documentation for new or modified features or behaviors.
• [ ] a release note entry in the PR description.
• [ ] 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.