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.

Start with the “Functional and Class Components” section of the Component and Props documentation linked in the issue. Read the surrounding sections to understand which additional class-component features are explained later, then expand the comparison and motivation without duplicating those sections. Done means the page gives beginners a clearer explanation of the differences and why they matter.