more examples and explaination of overlaying

[Roger-luo]

from @jon-wurtz

Expand the concept of "overlaying" in the interpreter. I think this is related to extensibility (being able to compose interpreters together) but right now its a bit too short to be clear.

Yes, it is short because I think this is an advanced feature whose documentation can be improved after open-source. We can come back if we have extra time next week. But I'm not going to include it in the open-source milestone.