Fix comment on G29 to include 'Z'

Marlin/Marlin_main.cpp

    *
    * Enhanced G29 Auto Bed Leveling Probe Routine
    *
-   * Parameters With ABL_GRID:
+   * Parameters With LINEAR and BILINEAR:
    *
    *  P  Set the size of the grid that will be probed (P x P points).
    *     Not supported by non-linear delta printer bed leveling.
    *  L  Set the Left limit of the probing grid
    *  R  Set the Right limit of the probing grid
    *
+   * Parameters with BILINEAR only:
+   * 
+   *  Z  Supply an additional Z probe offset
+   *
    * Global Parameters:
    *
    * E/e By default G29 will engage the Z probe, test the bed, then disengage.