Fix usage, commentary of MANUAL_PROBE_START_Z, Z_AFTER_PROBING (#21692)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/src/gcode/bedlevel/mbl/G29.cpp

@@ -100,7 +100,11 @@ void GcodeSuite::G29() {
       // For each G29 S2...
       if (mbl_probe_index == 0) {
         // Move close to the bed before the first point
-        do_blocking_move_to_z(MANUAL_PROBE_START_Z);
+        do_blocking_move_to_z(0.4f
+          #ifdef MANUAL_PROBE_START_Z
+            + (MANUAL_PROBE_START_Z) - 0.4f
+          #endif
+        );
       }
       else {
         // Save Z for the previous mesh position
@@ -116,8 +120,14 @@ void GcodeSuite::G29() {
         _manual_goto_xy({ mbl.index_to_xpos[ix], mbl.index_to_ypos[iy] });
       }
       else {
-        // One last "return to the bed" (as originally coded) at completion
-        current_position.z = MANUAL_PROBE_HEIGHT;
+        // Move to the after probing position
+        current_position.z = (
+          #ifdef Z_AFTER_PROBING
+            Z_AFTER_PROBING
+          #else
+            Z_CLEARANCE_BETWEEN_MANUAL_PROBES
+          #endif
+        );
         line_to_current_position();
         planner.synchronize();