Better user documentation for generation chains:
* In the pool class properties table, note whether a pool class uses generational garbage collection.
* The arena has a default generation chain.
* The MPS_KEY_CHAIN keyword argument is now optional when creating an AMC, AMCZ or AMS pool.
* The pool classes AWL and LO now take an optional MPS_KEY_CHAIN keyword argument.
* The pool classes AMS, AWL and LO now take an optional MPS_KEY_GEN keyword argument specifying the generation for new objects.
* Remove material about generation chains from the user guide (just use the arena's default generation chain).