Document new argument.

biosimulators_utils/sedml/validation.py

@@ -1717,12 +1717,13 @@ def validate_target(target, namespaces, context, language, model_id, model_etree
 
 def validate_target_xpaths(targets, model_etree, attr='id', separator=None):
     """ Validate that the target of each model change or variable matches one object in
-    an XML-encoded model and, optionally, return the value of one of its attributes
+    an XML-encoded model and, optionally, return the value of one of its attributes.
 
     Args:
         targets (:obj:`list` of :obj:`TargetGroupMixin`): model changes or variables
         model_source (:obj:`lxml.etree._ElementTree`): element tree for the XML model document
         attr (:obj:`str`, optional): attribute to get values of
+        separator (:obj:`str`, optional): string to use when combining attributes into a single id (i.e. 'J0.n' from 'J0' and 'n'.)  If None, don't combine and just use final id.
 
     Returns:
         :obj:`dict` of :obj:`str` to :obj:`str`: dictionary that maps each XPath to the