Cleaned up variable spindle output (PWM). Code and config comments.

- Variable spindle speed output as a configuration option. Thanks
@EliteEng! When enabled, the Z-limit (D11) and spindle enable(D12) pins
switch to allow access to the hardware PWM output on pin D11.
Otherwise, everything should work as it does.

- Removed option for inverting the spindle and coolant enable pins.
This is a safety hazard, especially for the spindle. When Grbl
initializes, all pins are momentarily low until it finishes booting. If
an invert is enabled, this means the spindles can be energized briefly
during this time. If users need signal inversion, it’s recommended to
just wire in an inversion circuit instead.

- Cleared out references to spindle variable output in terms of step
signal. This isn’t complete and requires more deliberation before
installing.

- Cleared up and cleaned up some code and config comments.

config.h

@@ -86,10 +86,24 @@
 // acceleration, particularly noticeable on machines that run at very high feedrates, but may negatively
 // impact performance. The correct value for this parameter is machine dependent, so it's advised to
 // set this only as high as needed. Approximate successful values can widely range from 50 to 200 or more.
+// NOTE: Changing this value also changes the execution time of a segment in the step segment buffer. 
+// When increasing this value, this stores less overall time in the segment buffer and vice versa. Make
+// certain the step segment buffer is increased/decreased to account for these changes.
 #define ACCELERATION_TICKS_PER_SECOND 100 
 
-#define ADAPTIVE_MULTI_AXIS_STEP_SMOOTHING 
-#define ENABLE_SOFTWARE_DEBOUNCE
+// Adaptive Multi-Axis Step Smoothing (AMASS) is an advanced feature that does what its name implies, 
+// smoothing the stepping of multi-axis motions. This feature smooths motion particularly at low step
+// frequencies below 10kHz, where the aliasing between axes of multi-axis motions can cause audible 
+// noise and shake your machine. At even lower step frequencies, AMASS adapts and provides even better
+// step smoothing. See stepper.c for more details on the AMASS system works.
+#define ADAPTIVE_MULTI_AXIS_STEP_SMOOTHING  // Default enabled. Comment to disable.
+
+// Enables variable spindle output voltage for different RPM values. On the Arduino Uno, the spindle
+// enable pin will output 5V for maximum RPM with 256 intermediate levels and 0V when disabled.
+// NOTE: IMPORTANT for Arduino Unos! When enabled, the Z-limit pin D11 and spindle enable pin D12 switch!
+// The hardware PWM output on pin D11 is required for variable spindle output voltages.
+// #define VARIABLE_SPINDLE // Default disabled. Uncomment to enable.
+// #define SPINDLE_MAX_RPM 1000 // Max spindle RPM. This value is equal to 100% Duty Cycle on the PWM.
 
 // Minimum planner junction speed. Sets the default minimum junction speed the planner plans to at
 // every buffer block junction, except for starting from rest and end of the buffer, which are always
@@ -168,6 +182,14 @@
 // case, please report any successes to grbl administrators!
 // #define ENABLE_XONXOFF // Default disabled. Uncomment to enable.
 
+// A simple software debouncing feature for hard limit switches. When enabled, the interrupt monitoring
+// the hard limit switch pins will enable the Arduino's watchdog timer to re-check the limit pin state
+// after a delay of about 32msec. This can help with CNC machines with problematic false triggering of 
+// their hard limit switches, but it WILL NOT fix issues with electrical interference on the signal 
+// cables from external sources. It's recommended to first use shielded signal cables that are grounded
+// (old USB/computer cables work well) and wire in a low-pass circuit into each limit pin.
+// #define ENABLE_SOFTWARE_DEBOUNCE // Default disabled. Uncomment to enable.
+
 // ---------------------------------------------------------------------------------------
 
 // TODO: Install compile-time option to send numeric status codes rather than strings.