Add returns documentation to fill_between methods (matplotlib#9584)

Author: dstansby

lib/matplotlib/axes/_axes.py

@@ -4793,6 +4793,10 @@ def fill_between(self, x, y1, y2=0, where=None, interpolate=False,
  step : {'pre', 'post', 'mid'}, optional
  If not None, fill with step logic.
 
+ Returns
+ -------
+ `PolyCollection`
+ Plotted polygon collection
 
  Notes
  -----
@@ -4948,6 +4952,12 @@ def fill_betweenx(self, y, x1, x2=0, where=None,
  end points of the filled region will only occur on explicit
  values in the *x* array.
 
+
+ Returns
+ -------
+ `PolyCollection`
+ Plotted polygon collection
+
  Notes
  -----