DOCSP-11302 correct compound index stock vs location

Author: andf-mongodb

source/core/index-compound.txt

@@ -155,14 +155,23 @@ the following fields:
 - the ``item`` field *and* the ``location`` field,
 - the ``item`` field *and* the ``location`` field *and* the ``stock`` field.
 
-MongoDB can also use the index to support a query on ``item`` and
-``stock`` fields since ``item`` field corresponds to a prefix. However,
-the index would not be as efficient in supporting the query as would be
-an index on only ``item`` and ``stock``.
-
-However, MongoDB cannot use the index to support queries that include
-the following fields since without the ``item`` field, none of the
-listed fields correspond to a prefix index:
+MongoDB can also use the index to support a query on the ``item`` and
+``stock`` fields, since the ``item`` field corresponds to a prefix.
+However, in this case the index would not be as efficient in supporting
+the query as it would be if the index were on only ``item`` and
+``stock``. Index fields are parsed in order; if a query omits a
+particular index prefix, it is unable to make use of any index fields
+that follow that prefix.
+
+Since a query on ``item`` and ``stock`` omits the ``location`` index
+prefix, it cannot use the ``stock`` index field which follows
+``location``.  Only the ``item`` field in the index can support this
+query. See :ref:`create-indexes-to-support-queries` for
+more information.
+
+MongoDB cannot use the index to support queries that include the
+following fields since without the ``item`` field, none of the listed
+fields correspond to a prefix index:
 
 - the ``location`` field,