DOCS-362 capped collections behavior clarification

Author: Sam Kleinman

source/core/indexes.txt

@@ -94,7 +94,15 @@ is a 12-byte unique identifiers suitable for use as the value of an
    have an ``_id`` field by default. In 2.2, all capped collections
    have an ``_id`` field, except those in the ``local``
    :term:`database`. See the :ref:`release notes
-   <2.2-id-indexes-capped-collections>` for more information.
+   <2.2-id-indexes-capped-collections>` for more
+   information.
+
+   Additionally, MongoDB does not create indexes on capped collections
+   by default do not have an index on the ``_id`` field.  In the
+   course of replicating a capped collection, :term:`secondary`
+   members of the set will create an ``_id`` index. If you specify
+   ``autoIndexId: true`` when creating a capped collection MongoDB
+   will create an ``_id`` index on this collection.
 
 .. TODO fix the above when a full capped-collection page exists.