DOCS-489 aggregation operators and indexes

Author: kay-kim

source/applications/aggregation.txt

@@ -179,21 +179,22 @@ Because you will always call :method:`aggregate` on a
 the aggregation pipeline, you may want to optimize the operation
 by avoiding scanning the entire collection whenever possible.
 
-Aggregation Operators and Indexes
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Pipeline Operators and Indexes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Depending on the order in which they appear in the pipeline,
 aggregation operators can take advantage of indexes.
 
-The following aggregation operators:
+The following pipeline operators take advantage of an index when they
+occur at the beginning of the pipeline:
 
 - :agg:pipeline:`$match`
 - :agg:pipeline:`$sort`
 - :agg:pipeline:`$limit` 
 - :agg:pipeline:`$skip` 
 
-can take advantage of an index when placed at the **beginning** of the pipleline or
-placed **before** the following aggregation operators:
+The above operators can also use an index when placed **before** the
+following aggregation operators:
 
 - :agg:pipeline:`$project`
 - :agg:pipeline:`$unwind`