GH-50553: [Dev] Clarify invalid PR title guidance comment

[Reranko05]

Rationale for this change

The developer PR workflow automatically converts pull requests with invalid titles to drafts and posts a guidance comment. The current comment explains the required title format, but it doesn't explicitly state why the pull request was converted to a draft or that contributors should mark it as ready for review after fixing the title.

This change makes those steps clearer to contributors.

What changes are included in this PR?

• Explain that the pull request was automatically converted to a draft because its title doesn't match the required format.
• Instruct contributors to mark the pull request as ready for review after updating the title.

• GitHub Issue: [Dev] Improve invalid PR title workflow message to explain automatic draft conversion #50553

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[github-actions]

⚠️ GitHub issue #50553 has been automatically assigned in GitHub to PR creator.

[kou]

+1

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit 6b3a94c.

There were 3 benchmark results indicating a performance regression:

• Commit Run on test-mac-arm at 2026-07-21 07:11:43Z
  • tpch (R) with engine=arrow, format=parquet, language=R, memory_map=False, query_id=TPCH-11, scale_factor=1
  • tpch (R) with engine=arrow, format=parquet, language=R, memory_map=False, query_id=TPCH-12, scale_factor=1
• and 1 more (see the report linked below)

The full Conbench report has more details. It also includes information about 306 possible false positives for unstable benchmarks that are known to sometimes produce them.