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.

Comece pela seção “Functional and Class Components” da documentação “Component and Props” vinculada na issue. Leia as seções ao redor para entender quais recursos adicionais dos componentes de classe são explicados posteriormente e, em seguida, amplie a comparação e a motivação sem duplicar essas seções. O trabalho estará concluído quando a página explicar com mais clareza aos iniciantes as diferenças e por que elas são importantes.