docs: fix stale and missing native write details in iceberg-writes.md and iceberg.md

[andygrove]

What is the problem the feature request solves?

The Iceberg user guide has errors and gaps about native writes:

• docs/source/user-guide/latest/iceberg-writes.md:273-276 says float/double partition directories use Rust's shortest representation (f=1 where iceberg-java writes f=1.0). Since fix: render float and double Iceberg partition values like iceberg-java #5840 they use Java's Double.toString rules and match iceberg-java.
• iceberg-writes.md:316-319 says the parity suite compares committed manifests byte-for-byte. CometIcebergWriteActionSuite compares aggregated readable_metrics (min/max, counts), not manifest bytes.
• iceberg-writes.md does not say that writes whose input is a local relation, such as INSERT ... VALUES and df.writeTo(...).append() on a local DataFrame, need spark.comet.exec.localTableScan.enabled (default false). Without it they silently use iceberg-java even with both write flags on.
• iceberg-writes.md does not list Native Iceberg writer keeps a dictionary page for high-cardinality columns where iceberg-java writes none #6114 (dictionary page kept for high-cardinality columns) among the accepted divergences.
• docs/source/user-guide/latest/iceberg.md:184 lists "Iceberg writes (reads are accelerated, writes use Spark)" as unsupported. Native writes exist behind spark.comet.iceberg.write.enabled; that page should point to iceberg-writes.md.

Describe the potential solution

Fix the four points above. The "experimental, disabled by default" wording will need another pass when the defaults change (#5644).

Additional context

Found in an audit of the native Iceberg write path before enabling it by default. Part of #5649.