website: Document missing flag

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

@@ -31,6 +31,8 @@ The command-line flags are all optional. The list of available flags are:
 * `-backup=path` - Path to the backup file. Defaults to `-state-out` with
   the ".backup" extension. Disabled by setting to "-".
 
+* `-input=true` - Ask for input for variables if not directly set.
+
 * `-no-color` - Disables output with coloring.
 
 * `-refresh=true` - Update the state for each resource prior to planning