[ticket/11904] Update explanation for what Allow CDN does

PHPBB3-11904
Merge remote-tracking branch 'vsephpbb/ticket/11904' into develop-ascraeus

* vsephpbb/ticket/11904:
  [ticket/11904] Update explanation for what Allow CDN does
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/11904] Update explanation for what Allow CDN does