Clarify Documentation of THR_MDL_FAC

The documentation of the thrust model parameter `THR_MDL_FAC` did not
mention both thrust and "PWM" being relative values. Also the use of the
term PWM could be misleading, since the model is applicable to CAN ESCs
as well.

This commit rephrases the user documentation string and a few source
code comments, but no logic changes are made.

Closes PX4/Firmware#13105

src/modules/sensors/motor_params.c

@@ -55,10 +55,14 @@
 PARAM_DEFINE_FLOAT(MOT_SLEW_MAX, 0.0f);
 
 /**
- * Thrust to PWM model parameter
+ * Thrust to motor control signal model parameter
  *
- * Parameter used to model the relationship between static thrust and motor
- * input PWM. Model is: thrust = (1-factor)*PWM + factor * PWM^2
+ * Parameter used to model the nonlinear relationship between
+ * motor control signal (e.g. PWM) and static thrust.
+ *
+ * The model is: rel_thrust = factor * rel_signal^2 + (1-factor) * rel_signal,
+ * where rel_thrust is the normalized thrust between 0 and 1, and
+ * rel_signal is the relative motor control signal between 0 and 1.
  *
  * @min 0.0
  * @max 1.0