Convert all calls & docs references to check_plain() in core to Drupal\Component\Utility\String::checkPlain() in rest module

There's a current patch for this in #2089471: Convert all calls to check_plain() in core to Drupal\Component\Utility\String::checkPlain() in core/modules/M-Z (except system/simpletest), let's see how that gets on before splitting into tiny issues.