DOCS-618: clarification to colelction stats document

Author: Sam Kleinman

source/reference/collection-statistics.txt

@@ -86,8 +86,11 @@ Fields
 
 .. stats:: size
 
-   The size of the collection. The ``scale`` argument affects this
-   value.
+   The size of the data stored in this collection. This value does not
+   include the size of any indexes associated with the collection,
+   which the :stats:`totalIndexSize` field reports.
+
+   The ``scale`` argument affects this value.
 
 .. stats:: avgObjSize
 
@@ -110,9 +113,15 @@ Fields
 
 .. stats:: nindexes
 
-   The number of indexes on the collection. On standard, non-capped
-   collections, there is always at least one index on the primary key
-   (i.e. :term:`_id`).
+   The number of indexes on the collection. All collections have at
+   least one index on the :term:`_id` field.
+   
+   .. versionchanged:: 2.2
+      Before 2.2, capped collections did not necessarily have an
+      index on the ``_id`` field, and some capped collections created
+      with pre-2.2 versions of :program:`mongod` may not have an
+      ``_id`` index.
+      
 
 .. stats:: lastExtentSize