Enhance check for missing permissions on forks

[ocean90]

Don't show error for forks when using pull_request_target event. Update example to mention pull_request_target and how to down scope as necessary.

• https://docs.github.com/en/actions/learn-github-actions/events-that-trigger-workflows#pull_request_target
• https://docs.github.com/en/actions/security-guides/automatic-token-authentication#modifying-the-permissions-for-the-github_token