DOC: write docstring for the add motor test.

tests/test_rocket.py

@@ -400,6 +400,37 @@ def test_add_motor_coordinates(
     expected_motor_cdm,
     expected_motor_cpp,
 ):
+    """Test the method add_motor and related position properties in a Rocket
+    instance.
+
+    This test checks the correctness of the `add_motor` method and the computed
+    `motor_center_of_dry_mass_position` and `center_of_propellant_position`
+    properties in the `Rocket` class using various parameters related to the
+    motor's position, nozzle's position, and other related coordinates.
+    Different scenarios are tested using parameterization, checking scenarios
+    moving from the nozzle to the combustion chamber and vice versa, and with
+    various specific physical and geometrical characteristics of the motor.
+
+    Parameters
+    ----------
+    calisto_motorless : Rocket instance
+        A predefined instance of a Rocket without a motor, used as a base for testing.
+    cdm_position : float
+        Position of the center of dry mass of the motor.
+    grain_cm_position : float
+        Position of the grains' center of mass.
+    nozzle_position : float
+        Position of the nozzle.
+    coord_direction : str
+        Direction for coordinate system orientation;
+        it can be "nozzle_to_combustion_chamber" or "combustion_chamber_to_nozzle".
+    motor_position : float
+        Position where the motor should be added to the rocket.
+    expected_motor_cdm : float
+        Expected position of the motor's center of dry mass after addition.
+    expected_motor_cpp : float
+        Expected position of the center of propellant after addition.
+    """
     example_motor = SolidMotor(
         thrust_source="data/motors/Cesaroni_M1670.eng",
         burn_time=3.9,