docs: set --master to local[2] for the spark-shell/pyspark/spark-sql in quickstart examples

[rangareddy]

Describe the issue this Pull Request addresses

This PR standardizes the local development experience and prevents accidental connection to unintended clusters when using interactive Spark shells.

Currently, if the spark-shell, pyspark, or spark-sql scripts are executed without explicitly setting the --master flag, Spark may default to a potentially undefined or shared configuration. For common development workflows, it is safer and more efficient to guarantee a local, isolated environment.

Summary and Changelog

Summary:
Configures the default behavior of interactive Spark shell scripts (spark-shell, pyspark, spark-sql) to use a dedicated local master with two threads (local[2]).

Changelog:

• Modified shell scripts (spark-shell, pyspark, spark-sql) to include the --master local[2] configuration by default.
• Users can still override this setting by explicitly passing a different --master flag.

Impact

The impact is minor and primarily affects the development experience:

• User-facing Change: The default behavior of running spark-shell (and related interactive tools) without arguments will now always launch a 2-thread local instance, guaranteeing resource isolation for local testing.

• Performance Impact: None on actual cluster workloads. Local performance for interactive tasks may be marginally improved due to predictable resource allocation.

Risk Level

low

This change only affects non-cluster, interactive shell execution. It does not impact production configurations, cluster deployments, or submitted application jobs. The risk is limited to local developer environments.

Documentation Update

none

Contributor's checklist

• Read through contributor's guide
• Enough context is provided in the sections above
• Adequate tests were added if applicable