docs: correct what the Iceberg split-operator plan moves into AQE

[andygrove]

Which issue does this PR close?

No issue; documentation only. Follow-up to review feedback from @mbutrovich on the Comet 1.1.0 blog post (apache/datafusion-site#205), which repeated this wording.

Rationale for this change

Both Iceberg writes guides say the query feeding an Iceberg write can't be re-planned by AQE, and that the split-operator plan makes the write's input visible to AQE. That isn't right. Spark's InsertAdaptiveSparkPlan has case c: V2CommandExec => c.withNewChildren(c.children.map(apply)), in both Spark 3.5 and 4.0, so the input query of a V2 write such as AppendDataExec is already wrapped in AQE. What sits outside AQE is the write operator itself.

As #4658 describes it, the split moves the data-file writing step inside AQE, so it can be re-planned in response to its upstream operators, and separates it from the metadata writing and commit, which Comet does not accelerate.

What changes are included in this PR?

• user-guide/latest/iceberg-writes.md: the Overview now says AQE already re-plans the sub-query, that the write operator is what sits outside AQE, and that the split moves data-file writing inside AQE, apart from the commit.
• contributor-guide/iceberg-writes.md: the Split-Operator Plan section says the same and names InsertAdaptiveSparkPlan. The plan diagram now marks which operators run inside and outside AQE instead of labeling the input query "now visible to AQE and Comet".

How are these changes tested?

Documentation only. npx prettier@latest --check passes on both files.

[mbutrovich]

Thanks @andygrove!