docs: clarify parallel validation scope

[Abdeltoto]

Summary

• Clarify that --parallel/parallel=True runs independent Package resources or Inquiry tasks concurrently.
• Document that single-file/resource validation is not split across multiple processes.
• Tighten the CLI help and Resource.validate parameter docs to avoid implying single-resource multiprocessing.

Fixes #1721

Test plan

• python -m ruff check frictionless/console/common.py frictionless/resource/resource.py
• git diff --check