website: Document the -backup flag

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

@@ -21,6 +21,9 @@ execute a pre-determined set of actions.
 
 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" extention. Disabled by setting to "-".
+
 * `-no-color` - Disables output with coloring.
 
 * `-refresh=true` - Update the state for each resource prior to planning

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

@@ -21,6 +21,9 @@ for the configuration and state file to refresh.
 
 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" extention. Disabled by setting to "-".
+
 * `-destroy` - If set, generates a plan to destroy all the known resources.
 
 * `-no-color` - Disables output with coloring.

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

@@ -24,6 +24,9 @@ for the configuration and state file to refresh.
 
 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" extention. Disabled by setting to "-".
+
 * `-no-color` - Disables output with coloring
 
 * `-state=path` - Path to read and write the state file to. Defaults to "terraform.tfstate".