also improve docs

lib/galaxy/tool_util/xsd/galaxy.xsd

@@ -3968,14 +3968,18 @@ dataset for the contained input of the type specified using the ``type`` tag.
   <xs:complexType name="ParamOptions">
     <xs:annotation>
       <xs:documentation xml:lang="en"><![CDATA[
-
 See [/tools/extract/liftOver_wrapper.xml](https://github.com/galaxyproject/galaxy/blob/master/tools/extract/liftOver_wrapper.xml)
 for an example of how to use this tag set. This tag set is optionally contained
 within the ``<param>`` tag when the ``type`` attribute value is ``select`` or
-``data`` and used to dynamically generated lists of options. This tag set
-dynamically creates a list of options whose values can be
-obtained from a predefined file stored locally or a dataset selected from the
-current history.
+``data`` and used to dynamically generated lists of options.
+
+For data parameters it can only be used to restrict inputs to datasets having
+the same ``dbkey`` like another input by using a ``data_meta`` filter. See for
+instance here: [/tools/maf/interval2maf.xml](https://github.com/galaxyproject/galaxy/blob/master/tools/maf/interval2maf.xml)
+
+For select parameters this tag set dynamically creates a list of options whose
+values can be obtained from a predefined file stored locally or a dataset
+selected from the current history.
 
 There are at least five basic ways to use this tag - four of these correspond to
 a ``from_XXX`` attribute on the ``options`` directive and the other is to