Update What_Is_a_Font.md

1. Added a new sub section on "Serifs" as this is an early choice to make in font/type design. Once added will add a sub-section for "Brackets" that are for serifs.
2. Added some text in "terminals" section to mention serifs and clarify that serifs and terminals are not the same thing.
3. Added text for "Descenders" about how long descenders can be difficult to manage.