Document laziness in ophyd.

[danielballan]

We should write down in one place the different layers of laziness available, how to specify them, and when they are often useful.

[danielballan]

Key point from my notes that I don't want to lose:

When a Signal is instantiated it issues a connection request. I think--- but I need to check this -- that it does not block and wait for the connection to complete (i.e. it doesn't block waiting for a CreateChanResponse). Setting Component(Signal, ..., lazy=True) defers instantiation of the Signal object.