add docstring to from_config method in PSProcessor class

Author: SvenKlaassen

doubleml/utils/propensity_score_processing.py

@@ -132,6 +132,7 @@ def __init__(
 
     @classmethod
     def from_config(cls, config: PSProcessorConfig):
+        """Create PSProcessor from PSProcessorConfig."""
         return cls(
             clipping_threshold=config.clipping_threshold,
             extreme_threshold=config.extreme_threshold,