minor: making it clear that local.slaves exists in replica sets too

Author: Sam Kleinman

source/reference/local-database.txt

@@ -1,8 +1,8 @@
 .. _replica-set-local-database:
 
-==============
-Local Database
-==============
+======================
+The ``local`` Database
+======================
 
 .. default-domain:: mongodb
 
@@ -53,6 +53,10 @@ Collections on Replica Set Members
    This contains an object used internally by replica sets to track sync
    status.
 
+.. data:: local.slaves
+
+   This contains information about each member of the set.
+
 Collections used in Master/Slave Replication
 --------------------------------------------