DOC: clarify default window in rolling method (pandas-dev#18177)

Author: dstansby

doc/source/computation.rst

@@ -344,7 +344,9 @@ The following methods are available:
     :meth:`~Window.sum`, Sum of values
     :meth:`~Window.mean`, Mean of values
 
-The weights used in the window are specified by the ``win_type`` keyword. The list of recognized types are:
+The weights used in the window are specified by the ``win_type`` keyword.
+The list of recognized types are the `scipy.signal window functions
+  <https://docs.scipy.org/doc/scipy/reference/signal.html#window-functions>`__:
 
 - ``boxcar``
 - ``triang``

pandas/core/window.py

@@ -503,6 +503,9 @@ class Window(_Window):
     * ``general_gaussian`` (needs power, width)
     * ``slepian`` (needs width).
 
+    If ``win_type=None`` all points are evenly weighted. To learn more about
+    different window types see `scipy.signal window functions
+    <https://docs.scipy.org/doc/scipy/reference/signal.html#window-functions>`__.
     """
 
     def validate(self):