add a docstring for set_inference_steps

This explains the relation between first_step and strength,
as shown by @holwech here: #374

src/refiners/foundationals/latent_diffusion/model.py

@@ -33,6 +33,15 @@ def __init__(
         self.classifier_free_guidance = classifier_free_guidance
 
     def set_inference_steps(self, num_steps: int, first_step: int = 0) -> None:
+        """Set the steps of the diffusion process.
+
+        Args:
+            num_steps: The number of inference steps.
+            first_step: The first inference step, used for image-to-image diffusion.
+                You may be used to setting a float in `[0, 1]` called `strength` instead,
+                which is an abstraction for this. The first step is
+                `round((1 - strength) * (num_steps - 1))`.
+        """
         self.solver = self.solver.rebuild(num_inference_steps=num_steps, first_inference_step=first_step)
 
     @staticmethod