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

[anatoly techtonik]

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 at bugs.python.org>
<http://bugs.python.org/issue14979>
_______________________________________