update docs for new valid inputs.boolean values (#341)

flask_restful/inputs.py

@@ -200,7 +200,8 @@ def int_range(low, high, value, argument='argument'):
 
 
 def boolean(value):
-    """Parse the string "true" or "false" as a boolean (case insensitive). if
+    """Parse the string "true" or "false" as a boolean (case insensitive).
+    Also accepts "1" and "0" as True/False (respectively). If
     the input is from the request JSON body, the type is already a native
     python boolean, and will be passed through without further parsing."""
     if type(value) == bool: