SERVER-6131 Describe new behavior when $unwind is applied to missing and empty array values.

Author: astaple

source/reference/aggregation.txt

@@ -404,16 +404,15 @@ The current pipeline operators are:
         :pipeline:`$group` pipeline operators.
 
       - If you specify a target field for :pipeline:`$unwind` that
-        does not exist in an input document, the document passes
-        through :pipeline:`$unwind` unchanged.
+        does not exist in an input document, the input document is
+        ignored (generates no result documents).
 
       - If you specify a target field for :pipeline:`$unwind` that is
         not an array, :mongodb:func:`aggregate()` generates an error.
 
       - If you specify a target field for :pipeline:`$unwind` that
-        holds an empty array (``[]``), then that field is removed
-        from the result while all other fields are passed through
-        unchanged.
+        holds an empty array (``[]``) in an input document, the input
+        document is ignored (generates no result documents).
 
 .. pipeline:: $group