Include better documentation of options & G29 cmd

Roxy thoughtfully produced some really solid documentation for his
modified G29 and the integrated dynamic ABL parameters.

Marlin/Configuration.h

@@ -412,10 +412,18 @@ const bool Z_MAX_ENDSTOP_INVERTING = false; // set to true to invert the logic o
     #define BACK_PROBE_BED_POSITION 175
     #define FRONT_PROBE_BED_POSITION 50
 
-     // set the number of grid points per dimension
-     // I wouldn't see a reason to go above 3 (=9 probing points on the bed)
+    // set the max number of grid points per dimension
+    // The G29 command defaults to 3 if nothing is specified.  But setting the number of probed 
+    // points higher is very useful when getting a Bed Topology Report  (G29 n 5 T)
     #define AUTO_BED_LEVELING_GRID_POINTS 5
 
+    // Uncomment one of the following four lines so the Bed Topology Report can produce a map
+    // that relates accurately to your bed orientation.  
+
+    #define ORIGIN_FRONT_LEFT
+    //#define ORIGIN_BACK_LEFT
+    //#define ORIGIN_FRONT_RIGHT
+    //#define ORIGIN_BACK_RIGHT
 
   #else  // not AUTO_BED_LEVELING_GRID
     // with no grid, just probe 3 arbitrary points.  A simple cross-product

Marlin/Marlin_main.cpp

@@ -1623,15 +1623,51 @@ void process_commands()
       break;
 
 #ifdef ENABLE_AUTO_BED_LEVELING
+
+//
+// Enhanced G29 Auto Bed Leveling Probe Routine
+// 
+// This updated code provides a Bed Topology Report, Ability to control engagement and retraction of
+// Z-Probe and varying levels of verboseness.   It also allows the selection of the n x n grid at 
+// print time instead of being fixed at the time the firmware is built. It also handles probe retraction
+// issues for probes that require clearance much more gracefully.
+//
+// Parameter usage:
+//
+//    n (u & U)    Controls the size of the grid that will be probed.  Typical usage would be of 
+//            the form  G29 n 4   u & U were added because of deficiencies in the Repetier client
+//            not being able to use even a lower case n
+//            
+//    V        Controls the verbose level.  Typical usage would be of the form   G29 V 3
+//
+//    T        Controls generation of the Bed Topology Report.  Typical usage would be of the
+//            form   G29 n 5 T    to get a detailed map of your bed.   This is very useful
+//            for manual bed leveling as well as knowing where imperfections exist on your
+//            bed (to assist with part placement)
 //
-// Enable one of these defines to configure a topographical map to be printed for your bed:
+//    E        Controls engagement and retraction of the Z-Probe.  It Defaults to leaving the
+//            probe engaged during the probing of the n x n grid.  This helps the repeatability
+//            of the Z-Probe and produces better Auto Bed Leveling results.  Typical usage 
+//            would be of the form   G29 E
 //
-//#define ORIGIN_BACK_LEFT
-//#define ORIGIN_FRONT_RIGHT
-//#define ORIGIN_BACK_RIGHT
-#define ORIGIN_FRONT_LEFT
+//  These following 4 parameters are typically generated by a Post Processing script 
+//  configured within your slicer to process the generated GCode.   Currently Curra and 
+//  Slic3r are supported.   The Post Processing scripts for these slicers will determine the 
+//  size of the print on the bed and limit the Grid Probe to the required region in order to
+//  increase the bed leveling accuracy.  Please not that these position parameters assume
+//  Left is the smallest X-Axis coordinate, Right is the larger X-Axis coordinate and Front is
+//  the smallest Y-Axis coordinate and Back is the larger Y-Axis coordinate.
 //
+//    L      Controls left side grid probe position.  If unspecified, will use  LEFT_PROBE_BED_POSITION
 //
+//    R      Controls the right side grid probe position.   If unspecified, will use  RIGHT_PROBE_BED_POSITION
+//
+//    F      Controls front side grid probe position.  If unspecified, will use  FRONT_PROBE_BED_POSITION
+//
+//    B      Controls the back side grid probe position.   If unspecified, will use  BACK_PROBE_BED_POSITION
+//
+//
+
     case 29: // G29 Detailed Z-Probe, probes the bed at 3 or more points.
 		// Example Syntax:  G29 N 4 V 2 E T
         {