adding comment about required version of mongodb to use the roles param

library/database/mongodb_user

@@ -64,6 +64,7 @@ options:
         version_added: "1.3"
         description:
             - The database user roles valid values are one or more of the following: "read", "readWrite", "dbAdmin", "userAdmin", "clusterAdmin", "readAnyDatabase", "readWriteAnyDatabase", "userAdminAnyDatabase", "dbAdminAnyDatabase"
+            - This param requires mongodb 2.4+
         required: false
         default: "readWrite"
     state: