document attribute_value_filter as deprecated

lib/galaxy/tool_util/xsd/galaxy.xsd

@@ -3622,8 +3622,9 @@ These values are defined in the module
 [/lib/galaxy/tools/parameters/dynamic_options.py](https://github.com/galaxyproject/galaxy/blob/master/lib/galaxy/tools/parameters/dynamic_options.py)
 in the ``filter_types`` dictionary. 
 
+Deprecated filter types:
 
-* ``attribute_value_splitter``: 
+* ``attribute_value_splitter`` 
         ]]></xs:documentation>
       </xs:annotation>
     </xs:attribute>

lib/galaxy/tools/parameters/dynamic_options.py

@@ -335,6 +335,8 @@ class AttributeValueSplitterFilter(Filter):
     """
     Filters a list of attribute-value pairs to be unique attribute names.
 
+    DEPRECATED: just replace with 2 rounds of MultipleSplitterFilter
+
     Type: attribute_value_splitter
 
     Required Attributes: