Follow up RSC node renderer test guide copy-paste hazards

[justin808]

Follow-up from merged PR #3223 and review comment #3223 (comment).

PR #3223 added the RSC + node-renderer system test recipe. The merged guide is usable, but a few review notes remain worth handling together because they affect copy-paste safety and reader scanability.

Recommended updates

• Clarify the Minitest story. The current intro says Minitest users can reuse the ENV setup and ReactOnRails::TestHelper.ensure_assets_compiled, but must provide their own suite-level renderer lifecycle harness. Either add an abbreviated Minitest lifecycle recipe or explicitly state that the renderer lifecycle section is RSpec-only and a Minitest variant is future work.
• Promote the hook-ordering requirement near the Step 2 require_relative "support/rsc_node_renderer" line. The current Caveats bullet is accurate, but the ordering is critical enough to be visible before readers copy the snippet.
• Move the metatag warning before the Step 2 copy-paste block and add an immediately actionable narrowed-regex example such as spec/(system/rsc|features/rsc).
• Add a short .gitignore reminder for generated test-only paths:
  • tmp/node-renderer-bundles-test-*
  • log/node-renderer-test-*.log

Optional while touching the guide

• Mention that teams adopting the 3900 port range can check for existing services first, e.g. lsof -i :3900-3910.
• Either document the symlinked Rails.root/tmp limitation of the current start_with? guard or harden the example with Pathname#realpath.
• Consider warning on loopback Errno::ETIMEDOUT in the pre-spawn port probe instead of silently proceeding, since it is unusual on 127.0.0.1.

Context

This is docs-only follow-up polish. It does not block the merged PR, but it should make the official RSC node-renderer test setup recipe safer for app authors to copy.