DOCS-383 clarification on buildIndexes=false

Author: Sam Kleinman

source/reference/replica-configuration.txt

@@ -93,6 +93,10 @@ Configuration Variables
    - index creation and maintenance overburdens the host
      system.
 
+   If set to ``false``, secondaries configured with this option *do*
+   build indexes on the ``_id`` field, to facilitate operations
+   required for replication.
+
 .. data:: members[n].hidden
 
    *Optional*.