updated docs

Author: forman

docs/guide.md

@@ -760,6 +760,11 @@ argument to your slice source.
     - `dict`: keyword arguments only;
     - Any other type is interpreted as single positional argument.
 
+You can also pass extra keyword arguments to your slice source using the 
+`slice_source_kwargs` setting. Its properties are passed as keyword arguments to 
+every slice source invocation. Keyword arguments passed as slice items with same name
+as in `slice_source_kwargs` take precedence.
+
 In addition, your slice source function or class constructor specified by 
 `slice_source` may define a 1st positional argument or keyword argument 
 named `ctx`, which will receive the current processing context of type