Add gcode documentation

Firmware/Marlin_main.cpp

@@ -5674,6 +5674,11 @@ if(eSoundMode!=e_SOUND_MODE_SILENT)
 
     /*!
 	### M20 - SD Card file list <a href="https://reprap.org/wiki/G-code#M20:_List_SD_card">M20: List SD card</a>
+    #### Usage
+    
+        M20 [ L ]
+    #### Parameters
+    - `L` - Reports ling filenames instead of just short filenames. Requires host software parsing.
     */
     case 20:
       SERIAL_PROTOCOLLNRPGM(_N("Begin file list"));////MSG_BEGIN_FILE_LIST