local: implement backend command "noop" for testing purposes

docs/content/local.md

@@ -424,4 +424,34 @@ See: the [encoding section in the overview](/overview/#encoding) for more info.
 - Type:        MultiEncoder
 - Default:     Slash,Dot
 
+### Backend commands
+
+Here are the commands specific to the local backend.
+
+Run them with with
+
+    rclone backend COMMAND remote:
+
+The help below will explain what arguments each command takes.
+
+See [the "rclone backend" command](/commands/rclone_backend/) for more
+info on how to pass options and arguments.
+
+These can be run on a running backend using the rc command
+[backend/command](/rc/#backend/command).
+
+#### noop
+
+A null operation for testing backend commands
+
+    rclone backend noop remote: [options] [<arguments>+]
+
+This is a test command which has some options
+you can try to change the output.
+
+Options:
+
+- "echo": echo the input arguments
+- "error": return an error based on option value
+
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