code: function endpointInvalidHeaders(endpoint) {
    const invalid = []
  
    for (const path of Object.keys(endpoint.headers)) {
      let pathIsUseful = false
      for (const useful of UsefulHeaders) {
        if (path === useful) {
          pathIsUseful = true
          break
        }
      }
      if (!pathIsUseful) invalid.push(path)
    }
  
    return invalid.length === 0 ? null : `${endpoint.path} (${invalid.join(', ')})`
  }
docs: endpoint => either null or "path (header1, header2, header3, ...)"