Add a little info about shard option of arc_tool, a=chris

diff --git a/en-US/pages/documentation.thtml b/en-US/pages/documentation.thtml
index f4dbf5e..f4c05e6 100755
--- a/en-US/pages/documentation.thtml
+++ b/en-US/pages/documentation.thtml
@@ -3129,6 +3129,10 @@ php arc_tool.php mergetiers bundle_name max_tier
 php arc_tool.php reindex bundle_name
 //reindex the word dictionary in bundle_name

+php arc_tool.php shard bundle_name generation
+    /* Prints information about the number of words and frequencies of words
+       within the generation'th index shard in the bundle */
+
 php arc_tool.php show bundle_name start num //outputs
 //items start through num from bundle_name
 //or name of non-Yioop archive crawl folder.