[docs] [issue14979] pdb doc: Explain how to extend debugger instead of sending readers to the source

anatoly techtonik added the comment: A minimal example of debugger is needed, with the explanation how frame.f_trace is used by debuggers. ---------- versions: +Python 3.4, Python 3.5 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue14979> _______________________________________