Improve Functional and Class Components in Component and Props

Just filling this issue after a tweet from @gaearon https://twitter.com/dan_abramov/status/993105470754705408

The Component and Props docs are a bit sparse on the differences between functional and class components (and the motivation behind those differences) but still mention that this difference exists and give an example of both: https://reactjs.org/docs/components-and-props.html#functional-and-class-components

While I personally am not sure the motivation/history behind class components is very useful to beginners it might be helpful to expand on the "additional features" beyond mentioning that they will be discussed in the next sections.

从 issue 中链接的 Component and Props 文档的“Functional and Class Components”部分开始。阅读周围的章节，了解后文解释了哪些额外的类组件功能，然后扩展比较和动机部分，但不要重复这些章节。完成的标准是：该页面能更清楚地向初学者解释差异以及它们为何重要。