Improve warning message for ignored property with attribute

[papafe]

An user on the forum had a property defined like the following:

    [PrimaryKey]
    private ObjectId _id { get; } = ObjectId.GenerateNewId();

That was raising the warning warning : Fody/Realm: SharedWith.Id has [MapTo], [PrimaryKey] applied, but it’s not persisted, so those attributes will be ignored.

The user did not understand what was the cause of this warning. We should probably improve the message on this error, to make it more clear about the reason for it.