Description

I think you should add an example that groups on multiple fields in 
https://docs.mongodb.com/manual/reference/operator/aggregation/group/

I ended up learning about this elsewhere, but the quality of those web pages was much lower than the quality of MongoDB docs, so I prefer to remain within the docs.

I wanted to do something like:

db.sales.aggregate( [ { $group: { _id: { v1,: "field1", v2: "field2" },
                        count: { $sum: 1 } } } ] )

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)