http://docs.python.org/library/pdb.html#pdb.Pdb Documentation for pdb says: "The debugger is extensible — it is actually defined as the class Pdb. This is currently undocumented but easily understood by reading the source." There should a link to the source.
Adding link to pdb source may be not be suitable. Readers may require to understand the states which pdb goes through. Docs here are better, IMO. -1 vote from me.
I agree that reading the source doesn't make it clear how to extend or use PDB, so I've changed the title. High level overview is required. I think an example would really help there. For instance a simple execution scroller - analogue of `python -m trace --trace <filename.py>`, but with PDB and play/pause buttons controlled from external script.