DOCS 13445 adding details to query plans page (#4770) (#4849)

* DOCS-13445 adding details on cached query plans

* DOCS-13445 adding details on cached query plans

Author: ltran-mdb2

source/core/query-plans.txt

@@ -20,7 +20,8 @@ For a query, the MongoDB query optimizer chooses and caches the most
 efficient query plan given the available indexes. The evaluation of the
 most efficient query plan is based on the number of "work units"
 (``works``) performed by the query execution plan when the query planner
-evaluates candidate plans.
+evaluates candidate plans. In general, the MongoDB query planner
+selects query plans with fewer ``works`` as the winning plan. 
 
 The associated plan cache entry is used for subsequent queries with the
 same query shape.
@@ -34,7 +35,8 @@ The following diagram illustrates the query planner logic:
 Plan Cache Entry State
 ~~~~~~~~~~~~~~~~~~~~~~
 
-Starting in MongoDB 4.2, the cache entry is associated with a state:
+Starting in MongoDB 4.2, each query shape is associated with one of three states
+in the cache:
 
 .. list-table::
    :header-rows: 1
@@ -49,13 +51,13 @@ Starting in MongoDB 4.2, the cache entry is associated with a state:
 
        No entry for this shape exists in the cache.
 
-       For a query, if the cache entry state for a shape is
+       For a query, if the cache entry state for a query shape is
        :ref:`Missing <cache-entry-missing>`:
 
        #. Candidate plans are evaluated and a winning plan is selected.
 
-       #. The selected plan is added to the cache in :ref:`Inactive
-          <cache-entry-inactive>` state with its ``works`` value.
+       #. The cache creates an entry for the query shape in state :ref:`Inactive
+          <cache-entry-inactive>` with its ``works`` value. 
 
    * - :ref:`Inactive <cache-entry-inactive>`