Add pros and cons of polymorphic variants versus normal variants

[feihong]

Source: #52 (comment)

It seems increasingly unlikely that all those points can be covered in just a single chapter.

As part of this,

• Need to validate my usage of polyvars as "symbols" in Discount.
• Also replace mentions of "variant" with "normal variant", for example in https://react-book.melange.re/promo-codes/#variants-vs-polymorphic-variants