Clarify when "string" should be used (and be escaped), and when
"template.HTML" should be used (no need to escape)
And help PRs like #29059 , to render the error messages correctly.
Change all license headers to comply with REUSE specification.
Fix#16132
Co-authored-by: flynnnnnnnnnn <flynnnnnnnnnn@github>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>