Add note explicitly marking Data.Typeable as not to be used

[kozross]

While we implicitly suggest that Data.Typeable is not for use by banning the DeriveDataTypeable pragma, the only place we say this (and implicitly at that) is the justification section for pragmata. As this section is long, and covers a lot of areas, it's easy to miss.

We should add an explicit ban on use of Data.Typeable except for legacy APIs, and recommend Type.Reflection instead.