Fix dead‑end explanation in pgr_extractVertices workshop docs

[akronim26]

Fixes #269

Changes proposed in this pull request:
Made the wording clear regarding a point in pgr_extractVertices docs.

@pgRouting/admins

Summary by CodeRabbit

• Documentation
  • Enhanced clarity in the Fill columns verification section with a more detailed explanation of why vertex count may not match the total table rows, noting that some nodes function only as sources or targets.

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Walkthrough

A documentation paragraph in the "Verification" section of the Fill columns step is rewritten to clarify the concept of dead-end vertices. The revised text explains that some nodes serve only as sources or only as targets, preventing complete row updates in the vertices table.

Changes

Cohort / File(s)	Summary
Documentation clarification docs/basic/graphs.rst	Updated explanation in the "Verification" section to clearly describe why dead-end vertices prevent complete updates to all rows in the vertices table

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

• Single documentation file with straightforward text replacement
• No code logic, configuration, or behavior changes involved
• Content is clearly aligned with the linked issue requirements

Poem

🐰 A clearer path through workshop words we trace,
Dead ends explained with newfound grace,
Dead sources, lonely targets too,
Now readers know just what to do! ✨

Pre-merge checks and finishing touches

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically identifies the main change: fixing the dead-end explanation in pgr_extractVertices workshop documentation.
Linked Issues check	✅ Passed	The code changes directly address issue #269 by replacing the confusing dead-end explanation with the clarified wording suggested in the issue.
Out of Scope Changes check	✅ Passed	The pull request only modifies the documentation file (docs/basic/graphs.rst) to clarify the dead-end explanation, which is entirely within scope of the linked issue.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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📜 Recent review details

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Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 6114f86 and 7eb7b48.

📒 Files selected for processing (1)

• docs/basic/graphs.rst (1 hunks)

🔇 Additional comments (1)

docs/basic/graphs.rst (1)

230-230: Excellent clarity improvement.

The revision transforms an ambiguous statement into a clear explanation of why the UPDATE row counts differ from the total vertices. The new wording directly addresses reader expectations and effectively connects dead-end graph nodes (nodes with only source or only target roles) to the incomplete row updates.

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