docs: Require nullable for scoped model fields and fix example

[developerjamiu]

The SelectivelyHiddenClass example on the models page declares a scope=serverOnly field with a non-nullable type, which fails code generation. On a non-serverOnly class, any field with a scope other than all (serverOnly or none) must be nullable — enforced by the model analyzer since Serverpod 1.2.3.

• Fix the example: hiddenSecretKey: String → String?.
• Document the rule: a field with a scope other than all must be nullable.

Reported in the docs channel (the published example does not generate). Scoped to the current dev version; the same example exists in the versioned docs and can be backported separately if wanted.

[Zfinix]

So this has been in the docs since 2.0 right

[developerjamiu]

So this has been in the docs since 2.0 right

Yes