[lworld] Improve javadoc comments for various methods dealing with projection types

https://cr.openjdk.java.net/~briangoetz/valhalla/sov/02-object-model.html defines the "ref operator" and "unref operator" as follows:

"It will be useful to define the operator ref v to be a reference to v when v is an inline object, and v itself if it is already an object reference. Then ref is total on all representable values, and always returns a reference. (The opposite operator, unref, is partial, and applies only on references to inline objects, and the two form a projection-embedding pair.)"

As of the implementation for JDK-8237072, ref operator is partial and works only on values. This needs to be amended as it can result in simpler and cleaner code in places.