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.

從 issue 中連結的 Component and Props 文件的「Functional and Class Components」章節開始。閱讀周圍的章節，以了解後文說明了哪些額外的類別元件功能，然後擴充比較和動機，但不要重複這些章節。完成的標準是：該頁面能更清楚地向初學者解釋差異以及它們為何重要。