What does this mean and where can I find this requirement documented?Note that Collections of non-serializable types must not be used as mapping type.
In your own list of requirements for persistant entity fields, you don't mention any requirements about collection types being serializable:
The persistent fields or properties of an entity may be of the following types:
Java primitive types; java.lang.String; other Java serializable types (including wrappers of the primitive types, java.math.BigInteger, java.math.BigDecimal, java.util.Date, java.util.Calendar, java.sql.Date, java.sql.Time, java.sql.Timestamp, byte[], Byte[], char[], Character[], and user-defined types that implement the Serializable interface); enums; entity types; collections of entity types; embeddable classes; collections of basic and embeddable types.