Document that we filter blocks which are unconfirmed or too recent

[alokmenghrajani]

To ensure reproducible results, we filter some blocks:

When computing a balance:
Beancounter only computes balances up-to "current height" - 6 blocks. We discard the last 6 blocks + unconfirmed blocks.

When mapping a timestamp to a block number:
We can only find blocks in the range 6 to current height - 11 (because of the way we take the median).