Docs for SERVER-23991: $graphLookup should optimize with $match

Engineering Ticket Description:

If a pipeline has a $graphLookup followed by a $match, we should push as much of the $match as possible before the $graphLookup, as we do with $lookup and $match. Note that unlike $lookup, $graphLookup cannot internalize part of the $match and execute it on the foreign collection, as this could unintentionally prevent us from exploring part of the graph.