[DOCS] Mention new `GENERATED … AS IDENTITY` feature in the `Serial Types` page ?

I suggest adding a mention of `GENERATED … AS IDENTITY` as a replacement for `SERIAL` in the “Serial Types” section
8.1.4of the “Data Types” page. 

https://www.postgresql.org/docs/devel/static/datatype-numeric.html#datatype-serial

Postgres 10 sports a new `GENERATED … AS IDENTITY` for an identity column to effectively replace the `SERIAL`
pseudo-type.

See “PostgreSQL 10 identity columns explained” post for a discussion of the various issues that plague `SERIAL` but not
identitycolumns. 

https://blog.2ndquadrant.com/postgresql-10-identity-columns/

Seems a shame to let the naïve reader continue using `SERIAL` without knowing about identity column feature.

—Basil Bourque