Add more details around querying null/non-existent values

I've been using Mongoid for years and I _still_ find this confusing: if you have a field that is boolean, for example, how do you query for documents where the field is either not present, `null` or `false`?  I know part of this issue rests on the MongoDB query syntax being somewhat vague about how things like `.find({myfield: null})` should be handled.
