💡 Add G28 L description (MarlinFirmware#22144)

Marlin/src/gcode/calibrate/G28.cpp

@@ -195,9 +195,9 @@
  *  None  Home to all axes with no parameters.
  *        With QUICK_HOME enabled XY will home together, then Z.
  *
- *  O   Home only if position is unknown
- *
- *  Rn  Raise by n mm/inches before homing
+ *  L<bool>   Force leveling state ON (if possible) or OFF after homing (Requires RESTORE_LEVELING_AFTER_G28 or ENABLE_LEVELING_AFTER_G28)
+ *  O         Home only if the position is not known and trusted
+ *  R<linear> Raise by n mm/inches before homing
  *
  * Cartesian/SCARA parameters
  *
@@ -229,7 +229,7 @@ void GcodeSuite::G28() {
   #endif
 
   // Home (O)nly if position is unknown
-  if (!axes_should_home() && parser.boolval('O')) {
+  if (!axes_should_home() && parser.seen_test('O')) {
     if (DEBUGGING(LEVELING)) DEBUG_ECHOLNPGM("> homing not needed, skip");
     return;
   }