Add --beta option to documentation

README.asciidoc

@@ -267,6 +267,14 @@ instances.
         Downloads the update but does not apply it.  Only has effect
         if a staging directory is set.
 
+    `--beta=<betaname>`;;
+        Select the beta to install (use `--beta=public` to reset
+        to the `public` branch.  Note that `--validate` is required
+	when switching branches.
+
+    `--betapassword=<password>`;;
+        Use password to unlock beta (for password-locked betas).
+
 `cancelshutdown`::
     Cancels a pending update / shutdown / restart that was run with
     the `--warn` option

tools/arkmanager

@@ -3118,6 +3118,7 @@ showUsage() {
 	   --upstart          Use the Upstart arkmanager service to restart the server
 	   --no-autostart     Don't start the server after updating
 	   --warnreason=...   Specify an update reason other than the default
+	   --beta=...         Specify a beta to install (use --beta=public to reset)
 
 	Stop and restart commands take the following options:
 	   --saveworld        Saves world before shutdown