docs(batchprocessor): document data loss without retry/queue

[Smile232323]

Summary

• Document batch processor data-loss behavior when exporters reject data without retry/queue.
• Recommend enabling exporter retry_on_failure and/or sending_queue with links to exporterhelper docs.
• Add a short warning-log note for the rejection message and rejected_items count.

Testing

• Not run (docs-only change).

Refs #12443

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Smile232323 / name: Kaiiiiiiiii (6f16f92)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.85%. Comparing base (cce7dc1) to head (6f16f92).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14520   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files         677      677           
  Lines       42705    42705           
=======================================
  Hits        39226    39226           
  Misses       2425     2425           
  Partials     1054     1054           

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