Add comment for tez/tez_local mode

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/branch-0.15@1682148 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/conf/pig.properties b/conf/pig.properties
index 41976ae..27d846a 100644
--- a/conf/pig.properties
+++ b/conf/pig.properties
@@ -91,6 +91,8 @@
 #
 # * mapreduce (default): use the Hadoop cluster defined in your Hadoop config files
 # * local: use local mode
+# * tez: use Tez on Hadoop cluster
+# * tez_local: use Tez local mode
 #
 # exectype=mapreduce