Corrections to the inline documentation for role::graphite

* The number of Carbon cache instances is now six, not four.
* Removed TODO about automatically generating the list of destinations. I've
  thought about this before and was prompted to think about it again by
  I57d5d85bf. I agree that it's good to be DRY -- configurations that have to
  match in value should ideally be specified once, to avoid the possibility of
  error due to mismatching values. But I think the ways of achieving that here
  entail adding a layer of abstraction on the Graphite module, and that would
  come at the cost of diminished flexibility (if we design the module to
  generate configs for a single-node Graphite setup) or added complexity (if we
  design the module so that it is able to generate configuration files for
  single-node and multi-node setup), which would in turn mean a greater surface
  for bugs. So I don't think it's warranted.

  Note that this is the decision Graphite itself made: its config parser could
  also be made smart enough to automatically determine the set of caches a
  relay instance should forward to. But Graphite declines to be too clever
  about this, and I think it's the right call, to be honest.

Change-Id: Icc7fb9adb81eae13dcf532bbd25aede74d94289b
Merge "Corrections to the inline documentation for role::graphite" into production