[stzn/the-swift-programming-language-jp] New comment on pull request #420: if式とswitch式を追加、Expressionsの表記を原文に合わせる

[stzn/the-swift-programming-language-jp] Pull request closed: #420 if式とswitch式を追加、Expressionsの表記を原文に合わせる

[stzn/the-swift-programming-language-jp] Issue closed: #402 ifとswitchの説明追加