save_variables: Support saving variables to a disk file

Signed-off-by: Dushyant Ahuja dusht.ahuja@gmail.com
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/Config_Reference.md

@@ -1182,6 +1182,20 @@ gcode:
 #   Default is 0.
 ```
 
+## [save_variables]
+
+Support saving variables to disk so that they are retained across
+restarts. See
+[command templates](Command_Templates.md#save-variables-to-disk) and
+[G-Code reference](G-Codes.md#save-variables) for further information.
+
+```
+[save_variables]
+filename:
+#   Required - provide a filename that would be used to save the
+#   variables to disk e.g. ~/variables.cfg
+```
+
 ## [idle_timeout]
 
 Idle timeout. An idle timeout is automatically enabled - add an