Implement Driver CRUD API Spec for .explain()

With the Driver CRUD API spec implemented, we can now use the following command from the shell:

> db.test.updateMany( { "name" : "jason" }, { $set : { "name" : "json" } } )

However, we must use the old method for explaining the update:

> db.test.explain().update( { "name" : "jason" }, { $set : { "name" : "json" } }, { "multi" : true } )

.explain() should implement the Driver CRUD API for consistency