Clarifications to 'Resolved Values' section

[catamorphism]

I was re-reading the MessageValue example and noticed that it's easy to skip over what the last three methods are for, so I added a reference to the earlier text.

I also thought that the getValue() method could have another name, since it's confusing to think about the "value" that it returns, vs. the enclosing MessageValue. The name I thought of was getResult(), but I'm open to other suggestions. I also thought it needed a little more explanation.