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.

spindle_control.h

@@ -24,10 +24,13 @@
 
 #include <avr/io.h>
 
+// Initializes spindle pins and hardware PWM, if enabled.
 void spindle_init();
+
+// Sets spindle direction and spindle rpm via PWM, if enabled.
 void spindle_run(int8_t direction, uint16_t rpm);
+
+// Kills spindle.
 void spindle_stop();
-uint8_t spindle_pwm();
-void spindle_pwm_update(uint8_t pwm);
 
 #endif