Documentation needed for how we split jobs

We need good documentation on how and why we split jobs and how the settings
used for this affect this splitting.
As it is BJA and I have discovered flaws in the splitting strategy and
misunderstandings in both settings and splitting behaviour as well as
un-documented expections of how the harvest definition system should be used.
This can only become a bigger problem as we move towards open source.
NOTE: This bug is originally from Bugzilla bug_id=929.
This bug was previously assigned to Unassigned.