Add more references to serial() being outdated

[dmint789]

This page states:

🎉 PostgreSQL Identity Columns

[Source](https://wiki.postgresql.org/wiki/Don%27t_Do_This#Don.27t_use_serial): As mentioned, the serial type in Postgres is outdated and should be deprecated. Ideally, you should not use it. Identity columns are the recommended way to specify sequences in your schema, which is why we are introducing the identity columns feature

But the rest of the docs reference serial() all over the place. I was lucky to randomly stumble upon this notice when I was searching for info on generatedAlwaysAsIdentity(), but I could have very easily missed it. For example, this page has serial() used a ton, and I think it would be best to make it clear that it's best not to use it anymore.

[dmint789]

Also, what if I just need an integer that auto-increments? Because this makes it sound like serial() shouldn't be used in general, not just for primary keys.