Document PerfIO S3 client configuration

[gerashegalov]

Related to #15919.

Description

The PerfIO S3 reader supports selecting Netty or CRT with the existing
spark.rapids.perfio.s3.httpClient setting, but that setting was omitted from the generated
advanced configuration documentation. The Iceberg S3 async tuning settings are specific to
PerfIO and should not be declared as core RapidsConf entries.

This change:

• documents the existing PerfIO S3 HTTP client backend selector, including its supported values
  and default;
• documents the three new Iceberg S3 request timeout settings and updates the existing settings
  with backend-neutral descriptions where appropriate; and
• removes the PerfIO-only Iceberg S3 declarations from the core configuration registry while
  retaining their generated user documentation.

The existing configuration names and defaults are unchanged.

Validation:

• ./build/buildall --clean --profile=350 --module=dist --parallel=64 --option='-DskipTests'
• git diff --check

Performance testing is not required because this change only updates configuration metadata and
documentation; it does not alter request execution.

AI assistance: the implementation and validation were performed with Codex assistance and
reviewed by the author before publication.

Checklists

Documentation

• Updated for new or modified user-facing features or behaviors
• No user-facing change

Testing

• Added or modified tests to cover new code paths
• Covered by existing tests
  (Please provide the names of the existing tests in the PR description.)
• Not required

Performance

• Tests ran and results are added in the PR description
• Issue filed with a link in the PR description
• Not required

[greptile-apps]

Greptile Summary

This PR aligns the advanced configuration reference with PerfIO’s S3 client configuration and removes duplicate PerfIO-only entries from the core RapidsConf registry.

• Documents the Netty/CRT HTTP client selector and S3 timeout controls.
• Makes existing S3 descriptions backend-neutral where appropriate.
• Keeps configuration names and documented defaults unchanged while moving ownership outside the core registry.

Confidence Score: 5/5

The PR appears safe to merge; no concrete behavioral, documentation, security, or repository-rule issue remains.

The removed typed entries have no repository callers, and raw PerfIO settings continue to propagate through the prefix-filtered RAPIDS configuration map while configuration documentation is sourced through the generator’s local and classpath-provided metadata.

Important Files Changed

Filename	Overview
docs/additional-functionality/advanced_configs.md	Adds and updates advanced PerfIO S3 configuration documentation with backend selection, timeout defaults, and backend-neutral descriptions.
sql-plugin/src/main/scala/com/nvidia/spark/rapids/RapidsConf.scala	Removes unused PerfIO-specific Iceberg S3 declarations from the core configuration registry without changing raw configuration propagation.

_{Reviews (1): Last reviewed commit: "Document PerfIO S3 client configuration" | Re-trigger Greptile}