Documentation: Add site-path arg to reindex command

Change-Id: I55b04100008e62156bf314dd5a607a6b298e1539
diff --git a/Documentation/pgm-reindex.txt b/Documentation/pgm-reindex.txt
index e13d518..5167277 100644
--- a/Documentation/pgm-reindex.txt
+++ b/Documentation/pgm-reindex.txt
@@ -7,6 +7,7 @@
 [verse]
 --
 _java_ -jar gerrit.war _reindex_
+  -d <SITE_PATH>
   [--threads]
   [--changes-schema-version]
   [--verbose]