docs: update Scala comments that reference the removed iceberg-writes.md page #7101

What is the problem the feature request solves?

#7090 folds docs/source/user-guide/latest/iceberg-writes.md into the Writes section of iceberg.md (headings demoted one level, so the anchors are unchanged). Four Scala comments still name the old page. #7090 leaves them alone so the docs-only change does not trigger the Scala CI tiers:

• spark/src/test/scala/org/apache/spark/sql/benchmark/CometIcebergEndToEndBenchmark.scala:64 and spark/src/test/scala/org/apache/spark/sql/benchmark/CometIcebergWriteBenchmark.scala:64 name docs/source/user-guide/latest/iceberg-writes.md, which no longer exists.
• spark/src/main/scala/org/apache/comet/serde/operator/IcebergWriteProtoTranslation.scala:202 and spark/src/test/scala/org/apache/comet/CometIcebergWriteActionSuite.scala:4517 say only iceberg-writes.md, which now resolves to the contributor-guide page rather than the user-guide section they mean.

Describe the potential solution

Point all four at docs/source/user-guide/latest/iceberg.md, naming the section where useful (for example "Accepted divergences" under Writes). This can ride along with the next PR that touches these files.

Additional context

Raised in review of #7090.