Clean up schema location doc re: prismaSchemaFolder

[stephenwade]

This note at the top:

If you are using the prismaSchemaFolder preview feature any files in the prisma/schema directory are detected automatically.

disagrees with this note later:

As of v6.6.0, you must always explicitly specify the location of your Prisma schema folder. There is no "magic" detection of the Prisma schema folder in a default location any more.

Remove the note that says the schema folder will be detected automatically.