Fix erroneous return values in docs

src/oemof/solph/_energy_system.py

@@ -178,11 +178,9 @@ def __init__(
     def _extract_periods_years(self):
         """Map years in optimization to respective period based on time indices
 
-        Returns
-        -------
-        periods_years: dict
-            year of the start of each period,
-            relative to the start of the optimization run and starting with 0
+        Attribute `periods_years` of type list is set. It contains
+        the year of the start of each period, relative to the
+        start of the optimization run and starting with 0.
         """
         periods_years = [0]
         if self.periods is not None:
@@ -195,14 +193,11 @@ def _extract_periods_years(self):
 
     def _extract_periods_matrix(self):
         """Determines a matrix describing the temporal distance to each period.
+
+        Attribute `periods_matrix` of type list np.array is set.
         Rows represent investment/commissioning periods, columns represent
         decommissioning periods. The values describe the temporal distance
         between each investment period to each decommissioning period.
-
-        Returns
-        -------
-        period_distance_matrix: np.array
-
         """
         periods_matrix = []
         if self.periods is not None: