Review Observe handler in the algorithms

[relu91]

Note: the observe handler is not mentioned later on in the algorithm. Also, I think the observe handler now has another function: it is called when somebody observes the property.

Originally posted by @relu91 in #450 (comment)

[relu91]

I think the problem still remains. The introduction text still mentions some interaction with the ObserveHandler when emitting property changes. I'm labelling as a bug.

[zolkis]

We have internal slot for observe handles. We can include a reference with the prose.