docs: improve COMMENT diagram readability

[ebembi-crdb]

Demo: SQL Diagram Automation

This PR demonstrates a typical writer workflow - improving diagram readability.

The Change

In the COMMENT statement diagram, replace the internal grammar term 'SCONST' with the user-friendly label comment_text.

Before: The diagram shows SCONST (string constant - confusing for users)
After: The diagram shows comment_text (clear and descriptive)

What This Demonstrates

1. Single file edit - Only diagrams.go changed (no Bazel files!)
2. Automatic CI - Workflow triggers on diagrams.go changes
3. PNG previews - See the updated diagram right in this PR
4. Validation - Broken links, naming conflicts caught before merge
5. Auto-sync - Bot creates PR in generated-diagrams repo

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This is a demo PR showcasing the SQL diagram automation workflow.

[github-actions]

SQL Diagram Generation Report

✅ Validation Passed

🔄 Diagram changes detected
A PR has been opened to sync these changes: cockroachdb/generated-diagrams#26

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This comment was generated by the SQL Diagram CI workflow.