[SPARK-4140] Document dynamic allocation

Once the external shuffle service is also documented, the dynamic allocation section will link to it. Let me know if the whole dynamic allocation should be moved to its separate page; I personally think the organization might be cleaner that way.

This patch builds on top of @oza's work in #3689.

@aarondav @pwendell
It would be nice to add a short clause explaining why this is the case