Explicit filter by dist column for efficiency

## Communication goals (e.g. detailed howto vs orientation)

When filtering by a column on which the distribution column functionally depends, the query will go faster if you also include the distribution column in the filter. This won't change the results, it merely allows Citus to prune shards efficiently.

### Good locations for content in docs structure

Section in http://docs.citusdata.com/en/v7.5/performance/performance_tuning.html about writing fast select statements.