docs: Document the QUAD_GANTRY_LEVEL command in G-Codes.md

Signed-off-by: Kevin O'Connor <[email protected]>

docs/G-Codes.md

@@ -1067,6 +1067,21 @@ CYCLE_TIME parameter is not stored between SET_PIN commands (any
 SET_PIN command without an explicit CYCLE_TIME parameter will use the
 `cycle_time` specified in the pwm_cycle_time config section).
 
+### [quad_gantry_level]
+
+The following commands are available when the
+[quad_gantry_level config section](Config_Reference.md#quad_gantry_level)
+is enabled.
+
+#### QUAD_GANTRY_LEVEL
+`QUAD_GANTRY_LEVEL [RETRIES=<value>] [RETRY_TOLERANCE=<value>]
+[HORIZONTAL_MOVE_Z=<value>] [<probe_parameter>=<value>]`: This command
+will probe the points specified in the config and then make
+independent adjustments to each Z stepper to compensate for tilt. See
+the PROBE command for details on the optional probe parameters. The
+optional `RETRIES`, `RETRY_TOLERANCE`, and `HORIZONTAL_MOVE_Z` values
+override those options specified in the config file.
+
 ### [query_adc]
 
 The query_adc module is automatically loaded.