Document how to correctly use @default for JSON columns

[SevInf]

JSON values in @default() should be quoted:

history Json @default("[]")

See prisma/prisma#22063.

That makes sense with the way JSON is handled one the engine side, however, for TS users this is not as obvious, as they always receive JSON as a parsed values. I haven't found explicit mention of this in either JSON or @default documentation.

[applemate]

I tried this, and the default value still null. Not the array we wanted.

[nikolasburk]

Created the PR for this here: #6121