add docstring

Signed-off-by: Victor Garcia Reolid <victor@seita.nl>

flexmeasures/data/models/generic_assets.py

@@ -139,6 +139,7 @@ def asset_type(self) -> GenericAssetType:
 
     @property
     def offspring(self) -> list[GenericAsset]:
+        """Returns a flattened list of all offspring, which is looked up recursively."""
         offspring = []
 
         for child in self.child_assets: