DOC: Clarify default window-safety for Filters.

zipline/pipeline/filters/filter.py

@@ -167,8 +167,8 @@ class Filter(RestrictedDTypeMixin, ComputableTerm):
     output of a Pipeline and for reducing memory consumption of Pipeline
     results.
     """
-
-    # make filters window safe
+    # Filters are window-safe by default, since a yes/no decision means the
+    # same thing from all temporal perspectives.
     window_safe = True
 
     ALLOWED_DTYPES = (bool_dtype,)  # Used by RestrictedDTypeMixin