heater_sim.c: Note that the heater isn't implemented in the simulator.
dda_maths.h: Add comment on units of C0.
dda_lookahead.c: Eliminate debug crossF 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, I, and D parameters.
config.default.h: Add DEBUG_LED_PIN to the pinout section.
config.*.h: extend DEBUG_LED_PIN comment to all config templates.