Rewrite of descriptor section

I rewrote the descriptor example and prose.

The previous Meter/Foot example, as written, stored the state in the descriptor object rather than the instances of the Distance class. This means that all Distances would have the same state, which is clearly undesirable. I also got rid of the meter descriptor, turning it into a regular attribute and making feet a view over meters. I think this more clearly illustrates how descriptors can be used.

I changed the prose to be more precise, trying not to increase verbiage.