Update docs

Author: Kirill Shirinkin

website/source/docs/commands/output.html.markdown

@@ -21,4 +21,8 @@ current directory for the state file to query.
 The command-line flags are all optional. The list of available flags are:
 
 * `-state=path` - Path to the state file. Defaults to "terraform.tfstate".
-
+* `-module=module_name` - The module path which has needed output.
+    By default this is the root path. Other modules can be specified by
+    a period-separated list. Example: "foo" would reference the module
+    "foo" but "foo.bar" would reference the "bar" module in the "foo"
+    module.