What does ids.num-partitions do?

I know there's the [documentation ](http://docs.janusgraph.org/0.1.0-SNAPSHOT/config-ref.html) but it's unclear how it relates to `ids.block-size`. Can someone add more colour to the documentation please?

Better explanation of using `ids.block-size` to reserve blocks of ids by each instance could be better explained too. Maybe detail somewhere what Janus does at startup etc? I'm not sure I've see that anywhere (although could have missed it).