docs: improve docstring

src/qibolab/_core/instruments/qm/controller.py

@@ -346,10 +346,16 @@ def register_pulses(self, configs: dict[str, Config], sequence: PulseSequence):
     def register_duration_sweeper_pulses(
         self, args: ExecutionArguments, configs: dict[str, Config], sweeper: Sweeper
     ):
-        """Register pulse with many different durations.
+        """Register pulses needed to implement a duration sweep.
 
-        Needed when sweeping duration. When interpolation is used, only
-        a single pulse with the minimum duration is used.
+        For standard (non-interpolated) duration sweeps, we need to
+        upload distinct waveforms for every different duration.
+
+        When interpolation is used, we need to upload a single pulse
+        with the minimum duration of the sweeper, because the QM real-
+        time interpolation can only stretch and not compress arbitrary
+        waveforms, as documented in
+        https://docs.quantum-machines.co/latest/docs/Guides/features/?h=interpo#dynamic-pulse-duration
         """
         for pulse in sweeper.pulses:
             if isinstance(pulse, (Align, Delay)):
@@ -369,6 +375,7 @@ def register_duration_sweeper_pulses(
                     ids[0], configs[ids[0]], sweep_pulse
                 )
             else:
+                assert sweeper.parameter is Parameter.duration
                 for value in values:
                     sweep_pulse = original_pulse.model_copy(update={"duration": value})
                     sweep_op = self.register_pulse(ids[0], configs[ids[0]], sweep_pulse)