fix(mc_pos_control): allow disabling trajectory time-stretch and fix docs

The trajectory time-stretch is skipped when the max-error param is at or below
FLT_EPSILON, so zero is a valid 'disabled' setting. Lower the minimum of
MPC_XY_ERR_MAX and MPC_Z_ERR_MAX from 0.1 to 0 so users can actually select it,
and document it in both params.

Also fix the setMaxAllowedVerticalError() doc comment, which incorrectly
claimed zero was the default: the MPC_Z_ERR_MAX param default is 1 m.

Signed-off-by: Julian Oes <julian@oes.ch>

src/lib/motion_planning/PositionSmoothing.hpp

@@ -333,9 +333,9 @@ public:
 
 	/**
 	 * @param error Maximum vertical error allowed by the trajectory generator. Set to param MPC_Z_ERR_MAX.
-	 * Zero (default) disables vertical time-stretch — trajectory integration is only slowed by horizontal
-	 * lag. A finite value makes the smoother slow down when the virtual trajectory leads the drone in Z,
-	 * preventing a noisy altitude reference from walking the commanded z away from the drone.
+	 * Zero disables vertical time-stretch — trajectory integration is only slowed by horizontal lag.
+	 * A finite value (param default is 1 m) makes the smoother slow down when the virtual trajectory leads
+	 * the drone in Z, preventing a noisy altitude reference from walking the commanded z away from the drone.
 	 */
 	inline void setMaxAllowedVerticalError(float error)
 	{

src/modules/mc_pos_control/multicopter_autonomous_params.yaml

@@ -123,10 +123,11 @@ parameters:
           trajectory is stopped to wait for the drone.
 
           This value can be adjusted depending on the tracking
-          capabilities of the vehicle.
+          capabilities of the vehicle. Set to 0 to disable the horizontal
+          time-stretch.
       type: float
       default: 2.0
-      min: 0.1
+      min: 0
       max: 10
       decimal: 1
       increment: 1
@@ -139,10 +140,11 @@ parameters:
           slowed down so the virtual setpoint can't walk away from the drone.
           This suppresses altitude overshoots caused by a noisy altitude
           reference (e.g. mission setpoint jittering as the home altitude
-          estimate is refined in flight).
+          estimate is refined in flight). Set to 0 to disable the vertical
+          time-stretch.
       type: float
       default: 1.0
-      min: 0.1
+      min: 0
       max: 10
       decimal: 1
       increment: 0.1