Document proc_open Windows /s /c change in PHP 8.0.0

[lacatoire]

Summary

• Update the Windows note in proc_open() to reflect that since PHP 8.0.0, the command is
  passed via %ComSpec% /s /c "$command" instead of %ComSpec% /c $command
• Add a changelog entry for PHP 8.0.0 documenting this breaking change
• This change was already documented in the migration 8.0 guide but was missing from the
  function page itself