[Doc][Misc] Clarify private-repo PR workflow and community PR rollback guidance#4

Copilot wants to merge 2 commits into

mainlHrHenry233/vllm-ascend-dev:mainfrom

copilot/remove-pr-from-vllm-ascendlHrHenry233/vllm-ascend-dev:copilot/remove-pr-from-vllm-ascendCopy head branch name to clipboard