python: doc args to OutputFormat filter()

Problem: There is no documentation for the args to the OutputFormat
filter() function.

Add inline documentation.

Author: chu11

src/bindings/python/flux/util.py

@@ -960,6 +960,9 @@ def filter(self, items):
 
         (`?+:` requests both actions: filter out field if it is empty for all
         items, if not expand to maximum width)
+
+        Args:
+            items (iterable): list of items to consider for output
         """
 
         #  Build a list of all format strings that have one of the width