DOCS-827 migrate query and nulls wiki

Author: kay-kim

source/faq/developers.txt

@@ -455,6 +455,9 @@ Different query operators treat ``null`` values differently:
 
   .. code-block:: javascript
 
+     { "_id" : 1, "cancelDate" : null }
+     { "_id" : 2 }
+
 - The ``{ cancelDate : { $type: 10 } }`` query matches documents that
   contains the ``cancelDate`` field whose value is ``null`` *only*;
   i.e. the value of the ``cancelDate`` field is of BSON Type ``Null``
@@ -471,13 +474,14 @@ Different query operators treat ``null`` values differently:
      { "_id" : 1, "cancelDate" : null }
 
 - The ``{ cancelDate : { $exists: false } }`` query matches documents
-  that that do not contain the ``cancelDate`` field:
+  that do not contain the ``cancelDate`` field:
 
   .. code-block:: javascript
 
      db.test.find( { cancelDate : { $exists: false } } )
 
-  The query returns only the document that contains the ``null`` value:
+  The query returns only the document that does *not* contain the
+  ``cancelDate`` field:
 
   .. code-block:: javascript
 

source/reference/operator/exists.txt

@@ -6,9 +6,13 @@ $exists
 
 .. operator:: $exists
 
-   *Syntax*: ``{ field: { $exists: boolean } }`` 
+   *Syntax*: ``{ field: { $exists: <boolean> } }``
 
-   :operator:`$exists` selects the documents that contain the field.
+   :operator:`$exists` selects the documents that contain the field if
+   ``<boolean>`` is ``true``. If ``<boolean>`` is ``false``, the query
+   only returns the documents that do not contain the field. Documents
+   that contain the field but has the value ``null`` are *not* returned.
+   
    MongoDB `$exists` does **not** correspond to SQL operator
    ``exists``. For SQL ``exists``, refer to the :operator:`$in`
    operator.
@@ -23,13 +27,6 @@ $exists
    where the ``qty`` field exists *and* its value does not equal either
    ``5`` nor ``15``.
 
-   .. note::
-
-      If set to ``false``, the query only returns those documents that
-      do not contain the field and *not* documents that contain the
-      field but has the value ``null``.
-
-
    .. seealso:: 
 
       - :method:`find() <db.collection.find()>`