Update comments on M125

Marlin/src/gcode/feature/pause/M125.cpp

@@ -37,9 +37,9 @@
  *       object. On resume (M24) the head will be moved back and the
  *       print will resume.
  *
- *       If Marlin is compiled without SD Card support, M125 can be
- *       used directly to pause the print and move to park position,
- *       resuming with a button click or M108.
+ *       When not actively SD printing, M125 simply moves to the park
+ *       position and waits, resuming with a button click or M108.
+ *       Without PARK_HEAD_ON_PAUSE the M125 command does nothing.
  *
  *    L = override retract length
  *    X = override X
@@ -75,6 +75,8 @@ void GcodeSuite::M125() {
   #endif
 
   if (pause_print(retract, park_point)) {
+    // SD Printing simply pauses, leaving the machine in a ready state,
+    // and can be resumed at any time, so don't wait in a loop here.
     if (!sd_printing) {
       wait_for_confirmation();
       resume_print();