Some of the sentences in Chapters 19 and 20 are re-translated:

- Fixed translation errors in Section 2 of Chapter 19 to prevent
  misleading readers;
- Retranslate some sentences to make the translation more clear and
  accurate.
Signed-off-by: Andy Deng <theandy.deng@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>