DOCS-10924: emphasize what happens when the field doesn't exist for $type

Author: schmalliso

source/reference/operator/aggregation/type.txt

@@ -98,6 +98,9 @@ Available Types
 
 .. include:: /includes/fact-bson-types.rst
 
+If the argument is a field that is missing in the input document,
+:expression:`$type` returns the string ``"missing"``.
+
 Example
 -------