G28 H Z, home Z at the current X/Y independent of POWER_LOSS_RECOVERY

"Fix, Improve Power-Loss Recovery (MarlinFirmware#22828)" enabled this H option, but
only when POWER_LOSS_RECOVERY is defined.  This is useful beyond power
loss recover, such as printing from OctoPrint, or any time there may
be a model at the normal safe x/y location and a Z home is needed.
It is also more flexible in letting the user choose the location based
on where the model was sliced for this print.  Add comments to
document H.

In my latest recovery I first needed the model height, home XY, home Z
(avoiding the model), move to the model, G30 to probe the model
height.  Then I could restart the print at that location.  Printing
from OctoPrint power loss recovery was not involved.

Note the probe to nozzle offset means some locations next to the edge
of the build plate will probe once, then fail because it sees the
location as unreachable and fail the home Z.

Author: dfries

Marlin/src/gcode/calibrate/G28.cpp

@@ -195,6 +195,11 @@
  *  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
+ *  H         Hold the current X/Y position when executing a home Z, or if
+ *            multiple axes are homed, the position when Z home is executed.
+ *            When using a probe for Z Home, positions close to the edge may
+ *            fail with position unreachable due to probe/nozzle offset.  This
+ *            can be used to avoid a model.  
  *
  * Cartesian/SCARA parameters
  *
@@ -486,7 +491,12 @@ void GcodeSuite::G28() {
           #endif
 
           #if ENABLED(Z_SAFE_HOMING)
-            if (TERN1(POWER_LOSS_RECOVERY, !parser.seen_test('H'))) home_z_safely(); else homeaxis(Z_AXIS);
+            // H means hold the current X/Y position when probing, if not
+            // specified, move to the define safe X/Y position before homing Z.
+            if (!parser.seen_test('H'))
+              home_z_safely();
+            else
+              homeaxis(Z_AXIS);
           #else
             homeaxis(Z_AXIS);
           #endif