simulator/heater_sim.c: Add notes that the heater isn't implemented in the simulator.
dda_maths.h: Add comment on units of C0.
dda_lookahead.c: Eliminate debug crossF uninitialized variable compile warning.

Fix:
  dda_lookahead.c:327:17: warning: 'crossF' may be used uninitialized in this function [-Wmaybe-uninitialized]
       sersendf_P(PSTR("Initial crossing speed: %lu\n"), crossF);
                 ^
gcode_process.c: Add comment on units of P - M130 , I - M131, and D M132 parameters.
config.default.h: Add DEBUG_LED_PIN to the pinout section.
config.*.h: extend DEBUG_LED_PIN comment to all config templates.