Plane: document INVERTEDFLT_CH
PID: added get_pid_4500()

this is a version of get_pid() that returns an int16_t constrained to
-4500 to 4500. This will prevent overflow errors for large PID gains
in ArduPlane and Rover
Plane: prevent overflow on large PID gains

this uses get_pid_4500() to prevent overflow of servo_out on large PID gains
Plane: use int16_t() not int() to ensure simulator matches AVR
Rover: use get_pid_4500() to prevent PID overflow

if users set much too large gains