DOCS-484 clarify using and the comma when operating on same field

Author: kay-kim

source/reference/operator/and.txt

@@ -39,15 +39,21 @@ $and
 
       db.inventory.find( { price: 1.99, qty: { $lt: 20 } , sale: true } )
 
-   If, however, a query requires an ``AND`` operation on the same
-   field, you *must* use the :operator:`$and` operator as in the
-   following example:
+   If, however, a query requires an ``AND`` operation on the same field
+   such as ``{ $price: { $ne: 1.99 } } AND { price: { $exists: true }
+   }``, then either use the :operator:`$and` operator for the two
+   separate expressions or combine the operator expressions for the
+   field ``{ $price: { $ne: 1.99, $exists: true } }``.
 
+   Consider the following examples:
+    
    .. code-block:: javascript
 
       db.inventory.update( { $and: [ { price: { $ne: 1.99 } }, { price: { $exists: true } } ] }, { $set: { qty: 15 } } )
 
-   This :method:`update() <db.collection.update()>` operation will set
+      db.inventory.update( { price: { $ne: 1.99, $exists: true } } , { $set: { qty: 15 } } )
+
+   Both :method:`update() <db.collection.update()>` operations will set
    the value of the ``qty`` field in documents where: 
 
    -  the ``price`` field value does not equal ``1.99`` **and** 

source/reference/operator/exists.txt

@@ -17,15 +17,11 @@ $exists
 
    .. code-block:: javascript
 
-      db.inventory.find( { $and: [ { qty: { $exists: true } }, { qty: { $nin: [ 5, 15 ] } } ] } )
+      db.inventory.find( { qty: { $exists: true, $nin: [ 5, 15 ] } } )
 
    This query will select all documents in the ``inventory`` collection
    where the ``qty`` field exists *and* its value does not equal either
    ``5`` nor ``15``.
-       
-   The above query used the :operator:`$and` operator because the query
-   performs an ``AND`` operation on the value of the same field and is
-   not specific to the :operator:`$exists` operator.
 
    .. seealso::