Add documentation about interstream deltas to the POD wiki

[corylown]

Interstream deltas were added via #1300. We should add documentation to the wiki explaining what these are and how to use them.

[cbeer]

I don't know how helpful it is, but this is the kind of sketch I used to keep track of what records should appear where:

I think the most confusing thing is the interstream delta should also includes the deletes from the previous stream (even though the record is also not present in the new stream). We need to do this because we don't know whether the harvester even pulled any of the intermediate deltas from the previous stream (and/or we'd have to keep all the previous stream deltas around)