docs: document changes to the virtual_sdcard module

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

docs/Config_Changes.md

@@ -6,6 +6,12 @@ All dates in this document are approximate.
 
 # Changes
 
+20200731:  The behavior of the `progress` attribute reported by
+the `virtual_sdcard` printer object has changed.  Progress is no
+longer reset to 0 when a print is paused.  It will now always report
+progress based on the internal file position, or 0 if no file is
+currently loaded.
+
 20200725: The servo `enable` config parameter and the SET_SERVO
 `ENABLE` parameter have been removed.  Update any macros to use
 `SET_SERVO SERVO=my_servo WIDTH=0` to disable a servo.

docs/G-Codes.md

@@ -60,6 +60,11 @@ Klipper also supports the following standard G-Code commands if the
 - Set SD position: `M26 S<offset>`
 - Report SD print status: `M27`
 
+In addition, the following extended commands are availble when the
+"virtual_sdcard" config section is enabled.
+- Load a file and start SD print: `SDCARD_PRINT_FILE FILENAME=<filename>`
+- Unload file and clear SD state:  `SDCARD_RESET_FILE`
+
 ## G-Code arcs
 
 The following standard G-Code commands are available if a "gcode_arcs"