docs: Add barrier function documentation and use it in KNN join example

[zhangfengcdt]

Address #68

[Copilot]

Pull Request Overview

This PR adds documentation for the barrier function and demonstrates its usage in KNN join scenarios to address optimization control needs. The documentation explains how to prevent filter pushdown and control predicate evaluation order in complex spatial joins.

• Adds a new "Optimization Barrier" section explaining the barrier function
• Provides a practical example showing barrier usage in KNN joins with filtering
• Demonstrates runtime evaluation control for complex spatial query optimization

Comments suppressed due to low confidence (1)

docs/reference/sql-joins.md:45

• Missing punctuation. Should be 'Example:' with a colon after 'Example'.

### Example For each city, find the 5 other closest cities.

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[jiayuasu]

LGTM overall. Can you elaborate a bit more in the doc about why applying the filter before and after the KNN actually have different semantic meanings?

[zhangfengcdt]

LGTM overall. Can you elaborate a bit more in the doc about why applying the filter before and after the KNN actually have different semantic meanings?

Sure, explanations have been added with examples to the doc.

[paleolimbot]

Thank you!