Improve discoverability of `Prisma.DbNull`/`Prisma.JsonNull`

[aqrln]

We should make the documentation about Prisma.DbNull and Prisma.JsonNull more discoverable.

The title of the section refers to "Filtering by null values", but the same also applies to create, insert and upsert operations (only Prisma.AnyNull is only used for filtering). This is, in fact, stated in the referenced section of the documentation, however it is hard to find.

This leads to confusion by our users who report expected and intended behavior as a bug: https://github.com/prisma/prisma/issues/9247

[tanberry]

Thanks @aqrln, yes we definitely need to improve this; I even had trouble finding it. @ObamaTheLlama114 (William on Docs team) and I might ping you with some questions soon.

[ObamaTheLlama114]

Background slack thread here