docs: missing part about multi-statements results

[abenhamdine]

Today, I was looking in the docs if multi-statements results have been implemented (I remembered all the conversation in #1190 ), but I didn't found anything.

This feature has been rolled out in pg 7.0 : https://github.com/brianc/node-postgres/blob/master/CHANGELOG.md#700

I would expect to find some doc in https://node-postgres.com/features/queries