docs: add HashJoinExec metrics to user guide

[fyrsta7]

Which issue does this PR close?

• Closes Include HashJoinExec in the metrics user-guide #19044.

Rationale for this change

HashJoinExec exposes several operator-specific metrics, but the metrics user guide did not document them yet. This PR follows the metrics documentation tracking issue by adding the HashJoinExec section.

What changes are included in this PR?

• Adds HashJoinExec operator-specific metrics to docs/source/user-guide/metrics.md.
• Adds a note near BuildProbeJoinMetrics to keep metric descriptions in sync with the user guide.
• Corrects the probe_hit_rate implementation comment to say "at least one match", matching the current metric accounting.

Are these changes tested?

This is a documentation/comment-only change with no runtime behavior change.

Checked locally:

• git diff --check
• cargo fmt --check
• npx prettier@2.7.1 --check docs/source/user-guide/metrics.md
• ./dev/rust_lint.sh

Are there any user-facing changes?

Yes. The metrics user guide now documents the existing HashJoinExec metrics. There are no runtime behavior or public API changes.

AI-assisted contribution note

This change is limited to documentation and comments. I verified the metric names and descriptions against BuildProbeJoinMetrics and HashJoinExec, and I do not have unknowns or assumptions to call out.