Add UNKNOWN_Z_NO_RAISE to example configs

Marlin/example_configurations/RigidBot/Configuration.h

@@ -780,6 +780,8 @@
 
 //#define NO_MOTION_BEFORE_HOMING  // Inhibit movement until all axes have been homed
 
+//#define UNKNOWN_Z_NO_RAISE // Don't raise Z (lower the bed) if Z is "unknown." For beds that fall when Z is powered off.
+
 //#define Z_HOMING_HEIGHT 4  // (in mm) Minimal z height before homing (G28) for Z clearance above the bed, clamps, ...
                              // Be sure you have this distance over your Z_MAX_POS in case.