Syslog CEF receiver: doc fixes and follow-up TODO from #2452

[utpilla]

Change Summary

Small documentation improvements and a follow-up TODO for the syslog CEF receiver, stemming from the review discussion on #2452 (comment).

• TODO for message fragmentation: Add a TODO noting that truncated messages currently emit the tail as a separate record without syslog header context. Links to the review discussion for future work on fragment-correlation metadata.
• Fix batching logic diagram: The ASCII diagram in README.md had misaligned borders — the max_batch_duration_ms line was wider than the box, causing the right-side | to not line up.
• Add developer-reference note: Add a note at the top of syslog-parsing-behavior.md clarifying it is a developer/contributor reference doc and pointing to the README for user-facing configuration.

What issue does this PR close?

• Closes #NNN

How are these changes tested?

Are there any user-facing changes?

No

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.18%. Comparing base (8e8dfcf) to head (f5e4456).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2461      +/-   ##
==========================================
- Coverage   88.19%   88.18%   -0.02%     
==========================================
  Files         604      604              
  Lines      214589   214589              
==========================================
- Hits       189249   189227      -22     
- Misses      24814    24836      +22     
  Partials      526      526              

Components	Coverage Δ
otap-dataflow	90.12% <ø> (-0.02%)	⬇️
query_abstraction	80.61% <ø> (ø)
query_engine	90.74% <ø> (ø)
syslog_cef_receivers	∅ <ø> (∅)
otel-arrow-go	52.44% <ø> (ø)
quiver	91.94% <ø> (ø)

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[utpilla]

@open-telemetry/arrow-maintainers

The failings tests are unrelated to the changes in this PR. I have created this issue to track them #2464