Hi
The correct BADI name is bbp_doc_save_badi
and the /SAPSRM/BD_WF_RESP_RESOLVER is used to determine the agent .
So after your process level step event gets triggered it will check the agent via this BADI
It is triggered all the times if the process level is approved or rejected
To me Easiest option seems to capture in DOC_SAVE
Let me know if that helps refer links below
http://help.sap.com/saphelp_srm70/helpdata/en/48/7df181bf5006b2e10000000a42189b/content.htm
https://help.sap.com/saphelp_srm70/helpdata/en/ad/d8fd25beb64bbe9d671f3199eb7edf/content.htm
Regards
Vinita