Allow @GeneratedValue for non-@Id fields

Hello,

My scenario is the following - I have a @MappedSuperclass which defines a field pk (which is generated value with a custom generic generator) but I also have an entity called Order with a property code. I have created a custom generic generator which calculates on the server what the order code must be, but even though I have specified it as this:

```
@Column(name = "code", nullable = false)
@GenericGenerator(name = "uuid", strategy = "uuid2")
@GeneratedValue(generator = "uuid")
private String code;
```

This generator is never called. I checked the javadoc and there explicitly says that the @GeneratedValue is used together with @Id. My proposal is to allow it to be used on columns that are not @Id.

Start with the @GeneratedValue javadoc and the issue's @MappedSuperclass and Order mapping example. Determine the specification and mapping changes needed to invoke a generator for a non-@Id field; done means the proposed behavior is defined and supported without changing the existing @Id contract.

Written by the indexing model from the issue text.